Basically any roguelike. They're almost all turn-based and generally take up almost no computer resources, so you can basically just keep them running in the background with negligible impact, even on a toaster. And even if you're like 'but I don't like roguelikes' there's an absolute metric ton of them with insane amounts of variation, so if you look hard enough you might find one with features you like.
DRL is inspired by Doom and is actually a fairly quick-paced and action oriented shooter type deal despite being a roguelike.
Cataclysm: DDA is an incredibly complex apocalypse survival game.
Elona+ is an excessively Japanese roguelike with a breadth of features including a fairly interesting character advancement system.
Gearhead and its sequel let you customize and pilot giant mecha and fight other giant mecha.
Dwarf Fortress hardly needs introduction here, but if you've never tried adventure mode, it's gotten pretty intense with the latest additions.
If just being roguelike-ish isn't in your interest, though, there's also tons of turn-based tactics and strategy games out there, including entire series of AAA games like Fire Emblem, Ogre Battle, Advance Wars, and Final Fantasy Tactics, most of which are old enough that they can be easily emulated.