Upgrade their PC to Windows 11 overnight
Click yes on all "recommended" options
Left-align taskbar and make it black
Restore their desktop wallpaper

I'm assuming that the haunting is because I had a vendetta against the haunted. In that case, I'd make their life miserable in the most subtle ways, including but not limited to:

• Move their furniture half an inch at random so they keep stubbing their toes.
• Make their clothing get stuck in doorhandles.
• Loosen the knobs on their drawers.
• Loosen the screws on the handles of their pots and pans by an eight of a turn.
• Shift things in their cupboards so that something falls out every time they open them.
• Damage the gaskets on their windows so that there's a draft and constant whistling sound.
• Reopen their resealable packages a little bit so the food spoils faster.
• Make their toilet leak, but only when they're sleeping.
• Pack lint into their phone's charging port so that their charger never plugs in snugly.
• Unbalance their appliances so they rattle/shake.
• Slightly unbalance their car tires.
• Loosen fasteners in their car so it produces annoying rattles.
• Make their pens leak.

Playing with their pets while they're sleeping, by throwing small objects around the house.

...I wouldn't be a scary ghost but at least I'd have my fun.

I would constantly log them out of all their social and turn off their phone — what a naughty ghost would I be :p

I wouldn’t. I’m a fucking ghost. I can just go about my day, knowing I’m a ghost, and not fuck with anyone.

My victims would never again find a matching pair of socks in their homes.

I'll befriend and play with their cats. Their cats will do a zoomie randomly.

Ohh wait that's how a cat normally works.

I'll make their cat miao randomly.

Wait that's how they works as well.

I'll stand at a corner of the room, attract their attention and makes them looks like they stare at that corner for no reason.

Ohh that's how they works too.

Ohh well i'll just stick with making dogs bark randomly.

I would haunt scientists until somebody proves that ghosts are real.

Demand to know "are we there yet?" while haunting their house. Because of your going to haunt somebody and scare them, you might as well confuse the hell out of them too.

Convince some kid that they have to spend a considerable amount of their lives, dedicated to figuring out why I'm here, and how they can figure out how to help me 'cross over'.

Then after a year or two I'd just get bored and tell them I made all of that up, and this is pretty much it. Then I'll go possess some guy to play videogames I guess.

I don't quite know what the mechanism for this would be, but I imagine giving people random boners/getting them randomly turned on at times where its not appropriate would be quite funny. There are few mundane experiences which are quite so unpleasant as arousal when it shouldn't happen

Float walk along behind them, invisible, while randomly farting, or making farting noises if ghost farts don't smell.

Constant unprompted calculus lessons

I would make sure that the toilet roll is the right way around.

As a ghost, do I have the power to visit people during their dream time?

Recite insurance commercial jinggles

show them random scenes from their lives, remind them of the choices they made.

I would manifest in their lives by ringing their windchimes when they are thinking of me.