If you like space games, X3 and X4 can probably eat up lots and lots of your time. Space Engineers if you're less interested in flying, space economics and creating a supply chain (which X lets you) and more in gathering materials and creating all sorts of weird, funny or actually amazing stuff. SE is probably better played with at least 1 other person, possibly more, while X is exclusively single player.

Grand Strategy games can also obliterate your free time. Civilization 5, any Total War game. Hell, Age of Empires 2 can have very long matches if you play on huge maps and people have loved that game for over 20 years.

You can also go for Fallout 4 or Skyrim with extra content mods, like dungeons, quests or areas.

Personally, I'm just past 400h on Palworld, so I'd recommend it as well. I've played from start to "finish" some 4 times by now. Official servers will be wiped sometime in December, possibly with a new patch arriving, but you can always play single player and even invite a friend to play on your local save and never lose progress (unless the save gets corrupted, which can happen).