Well you can pick up older CoD games on steam that should run on there, I don't think the newer ones will at all. Also I'll do my shameless if ya want to just shoot stuff Left 4 Dead 1 and 2 may be worth seeing if it will run. It's an old game about getting from point A to B while mindlessly killing tons of zombies. Can do it coop 4 players, has lots of custom maps. Just not sure if it will run on the graphics thing. Also Team Fortress 2 might be worth looking into. It's also free on Steam
A good platformer worth looking into would be Hollow Knight to see if it could run. It is hard though. Also they have Castlevania and Contra collections on steam if ya like retro side scrollers.
As for Sub Sims. Silent Hunter 3 and 4 are worth a look also. I don't know if you have them or have played them, but my dad absolutely loved them. He enjoyed 3 the most. There's also a new Sub Sim in early access, but not sure if it will run on your system and I cant remember the name of it off the top of my head.
3D platformer wise. I picked up one called Poi for my kid years ago and he loved it. Remember me of the old Super Mario 64.
Hope that helps a bit.