It Takes Two is a masterpiece for co-op gameplay and is great for casual gameplay. A Way Out was made by the same studio before It Takes Two and it's easy to see where they were able to improve on the experience, but it's also a great game for local co-op.

https://store.steampowered.com/app/1222700/A_Way_Out/

https://store.steampowered.com/app/1426210/It_Takes_Two/

https://store.steampowered.com/app/1225570/Unravel_Two/

https://store.steampowered.com/app/253230/A_Hat_in_Time/

https://store.steampowered.com/app/620/Portal_2/

https://store.steampowered.com/agecheck/app/476600/

https://store.steampowered.com/app/252110/Lovers_in_a_Dangerous_Spacetime/

https://store.steampowered.com/app/274190/Broforce/

https://store.steampowered.com/app/242550/Rayman_Legends/

https://store.steampowered.com/app/584400/Sonic_Mania/

https://store.steampowered.com/app/345180/Victor_Vran_ARPG/

https://store.steampowered.com/app/598550/HUNTDOWN/

https://store.steampowered.com/app/431940/Wildfire/

https://store.steampowered.com/app/360830/YookaLaylee/

https://store.steampowered.com/app/1016180/Terror_of_Hemasaurus/

https://store.steampowered.com/app/399820/Kopanito_AllStars_Soccer/

https://store.steampowered.com/app/61730/Critter_Crunch/

+1 for it takes two.

There’s also a pretty nice list already in the comments.

I do like a lot Portal 2 but wifey cannot play for too long with me as she becomes impatient. When that’s the case and she just want to watch a story like you would watch a movie, I play A Plague Tales (both of them) and she ends up wanting to know more and finally, crying.

It Takes Two is co-op masterpiece. It’s mostly a puzzle platformer but changes up the gameplay style every level. Lots of little mini games to get distracted by too. Can’t recommend it enough

LEGO games are fantastic

It Takes Two and A Way Out are co-op masterpieces

Not exactly a recommendation (most of what I would have said has been covered below) but, my partner and I use Co-Optimus to find new games to couch play together. Here's a filtered list of PC split-screen and couch co-op games.

https://co-optim.us/1m8Ysft

Hope that helps!

Me and my gf loved playing Divinity: Original Sin 2.

And we're super excited for Balder's Gate 3 made by the same Devs as DOS2. It's just had its release date move forward to Aug 3rd woo!

If you and your wife are into D&D-type stuff Divinity Original Sin 2 has local co-op. It's an isometric turn based adventure game. Super fun to play and on sale for about $18 at the moment, I think. I will say, it's a little more involved than some of the more casual games like Stardew, but if you're willing to put in a little time to learn the game it's one of the most rewarding games I've ever played.

My girlfriend and I actually played through the whole thing together!

Out of Space is a ton of fun, starts out hard, but stock with it and you can overcome.

Others I've seen mentioned already, but deserves more: Lovers in a Dangerous Spacetime and It Takes Two

We just played It Takes Two. Great variety of gameplay and level styles. Always something to learn, always something new happening, very different environments. Also, good story for husband and wife coop. Great game, I can recommend!

It’s not split screen, but an enjoyable game for two players in the same screen (so couch coop):

https://en.m.wikipedia.org/wiki/Spiritfarer

You play the new “Reaper” and his cat. Collect lost souls, that still have some issues to solve, travel the “in between” spirit world, build a house boat, take care of gardens, make music, etc. The artwork is cute, while the topics of the lost spirits can be unexpected sad. Still a great game to relax!

Shocked It Takes Two isn’t up here. Fiancé isn’t a gamer at all, but she LOVED this game. Brings us back to the great PS2 era 3D platformers.

Besides that, I’ll echo the other comments of Lego games (especially Lego Skywalker Saga)

A few that I've played:

20XX and 30XX - Megaman-style roguelites. Built with 2-player co-op in mind (either local or online) - when playing local, the screen space is shared but the screen zooms out when players are far apart (similar to Smash), so you still get about the same amount of view distance.
Hypercharge: Unboxed - FPS Horde Shooter/Tower Defence hybrid. Up to 4 players (local or online) have to defend cores from hordes of enemy toys, with the ability to place defences to make the job easier. It's mostly about the FPS shooter aspect over the tower defence stuff, though.
Streets of Rogue - GTA1-style sandbox roguelite. Up to 4 players (local or online). To describe Streets of Rogue is difficult. It's a very silly sandbox game where you're given a few objectives (usually neutralising folks or getting an item from another group of guys), but there's always many ways to go about things. It's also one of the few games where the 32 playable characters actually play wildly different from eachother (except the red/blue gangters but that's also the joke), and multiplayer adds another level on top of that as you can easily help (or hinder) eachother's Main Objective, as each class has their own goals.
Helldivers - Top-down shooter. Up to 4 players (local and online, at the same time) get dropped into a planet to complete various objectives. There's a lot of mechanics in this game which would probably kill other games (like how friendly fire is mandatory, or how reloading your gun discards the rest of your clip), but the devs (who also made Magicka, just fyi) made it work very well. There's a sequel coming later this year, too.
Sonic Robo Blast 2 - 3D Platformer. Up to 2 players local splitscreen, or 32 online can play together to run through co-op Sonic levels. Also the only game in this list that's completely free (by nature of it being a Sonic fangame).
A Hat in Time - 3D Platformer. Up to 2 players local splitscreen, and 50 online can play together in a platformer which I honestly find a lot more fun than Super Mario Odyssey. Also, Player 2 is an equal to the main hero this time instead of a glorified "little brother mode" - heck, Player 2 can actually trigger cutscenes and stuff without Player 1's involvement at all. Great stuff.
N++ - 2D Precision Platformer. If you've heard of Super Meat Boy, the original N is the game that inspired it. N++ is just a perfected version of N, with a brand new co-op mode with its own unique levels, which now requires two players to flawlessly move their way through levels where even a single slip-up means starting the level again (though that's fine - they're at max only 60 seconds long, but there's over a thousand of them)
Mercenary Kings - Metal Slug meets Monster Hunter. Up to 4 players local or online. Rise up the difficulty ranks by completing missions, upgrading the parts on your very modular guns and don't forget to eat before you go on a hunt- er, mission.
Pretty much the entirety of the side-scrolling beat-em-up genre, including Streets of Rage 4 (4 player local), River City Girls 1&2 (2 player local) and Teenage Mutant Ninja Turtles: Shredder's Revenge (6 player local and online, yes 6). This entire genre is based on local co-op, so just pick a game and dive in.

There are a lot of good suggestions here already. I want throw out Escape Academy. Split screen escape room game. My brother and I started playing it and had a blast in just an hour. Not sure about any story because we glossed over it, but we enjoy puzzles a lot.

Edit to include: Cuphead Wildermyth Overcooked 2 Terraria TMNT Shredders Revenge

It's not my favorite but I feel like it deserves to be mentioned: Alien Swarm.

Both Rayman games are really nice

Not "split screen" but "same screen":

Unraveled 2

Great for couples.

There's a mod that allows borderlands 2 to be split screen

It Takes Two.

Which reminds me, we really need to finish that game.

It’s not co op but head to head. But if you haven’t played duck game, I feel like you are missing out on some of the best local multiplayer pc has to offer.

If that’s too competitive, try Ultimate Chicken Horse - if you want some incentive watch a guy named Northernlion play it on YouTube

Next on the semi co op is Crawl although it’s still a little competitive, I can’t do it justice by describing it but it’s great, videos will do better to describe it.

If co-op is a must, I second divinity.

Really enjoyed “Heave Ho!” Co-op 2D arcade-style game where you play a character with two arms, can control the direction and grabbing of each arm, and must cooperate to climb/heave each other through obstacle course levels. Couch co-op, best with controllers.

My girlfriend and I both have computers (i built her a gaming PC so she could move off her laptop). The games we have enjoyed are Halo (no local co-op), Overcooked 2 (local co-op available), The Escapsists 2 (local co-op available), and "Keep talking and nobody explodes" (only 1 person needs the computer other is reading material helping them). Games we play together with co-op but we need to be on our own rigs are Apex legends (if we can call that Co-op), Starship troopers Extermination, Deep Rock Galactic, Left 4 dead 2, Satisfactory, The forest, Terraria, and Call of Duty BO3 custom zombies.

Honorable Mention: Old School Runescape. I grew up playing this game, played it again in college and got my girlfriend into it as well and now we both play often. It can be played on PC or even has a very well made mobile client that lets you play the same game on mobile. (be careful, the joke is that the game is called Run-Escape, since it tends to pull you in like a crack addiction lol)

It's not a split screen game, but it can be CO-OP. My partner and I play For the King (currently 75% off on steam).

Lovers in a Dangerous Spacetime!

Another not split screen but local co-op, up to 4 players.

If you're willing to get just a little technical you can use nucleus co-op (check out the sub-reddit for details) to split screen many, many games.

Castle Crashers and Guacamole 1/2, hands down

if you want some chaos; unrailed, overcooked, and plateup are good

A lot of great games mentioned so far. I’d like to add Streets of Rogue to the list. It’s a lot fun playing it together. Especially when things don’t exactly go to plan :)

How about firing Dolphin emulator and play some gamecube and wii games.

Divinity: Original Sin (and it's sequel) are two of my favorites that I played split screen with my wife. They're classic crpgs, so can be a bit slower paced, but both have great stories and are very good about allowing freedom to both people playing

Much like others, I can't say enough good things about It Takes Two.

The storyline, great! The gameplay, fantastic! The soundtrack, amazing!

That damn book, pissed me off to no end. (you'll understand that if you play it)

I'd put it up there in my top 10 games I've played of all time.

Grim Dawn is pretty great

Not split screen but my partner and I are really enjoying nine parchments. She's not a gamer at all, but the game on casual difficulty gives just the right amount of challenge for her.

My girlfriend and I thought It Takes Two was excellent. We also enjoyed A Way Out

Renegade Ops is a favorite of mine, with its 90's action movie vibe and explosions-filled gameplay. It's reminiscent of Desert Strike but more forgiving.

HELLDIVERS is another top-down shooter which borrows from Starship Troopers and features surprisingly engaging and deep mechanics.

Magicka and Magicka 2 are great! Can't recommend them enough. Both are on sale rn for like $2.50

The Goose Game, Bro Force, Diablo 3/4, Don't Starve, Moon Hunters. Just for a short list, I got more!

The goose game and moon hunters are easily two favorites. Peace was never an option.

Screencheet. It's the only fair combative experience.

Conflict Desert Storm, fun 4 player coop tactical third person shooter.