Bioshock - Good older PC game, most people know about and own this.
Bit Blaster XL - Fun little shooter for cheap, probably good for at least 2 hours of fun.
Curvatron - Cute puzzle/reaction time game. Like Tetris, but not at all, but kind've. Kinda.
Delver - Indie procedurally generated dungeon crawler. Good for a couple hours, not much fun after that.
Fairy Fencer F - Fun, cheap RPG with anime aesthetics. Love it/hate it gameply, like Neptunia.
Luftrausers - Again, cheap fun shooter. More substance than Bit Blaster, but feels less 'arcadey.'
Portal - You know portal. Puzzles, cake, killer robots.
Super Meat Boy - Skill based twitch platformer with tons of replayability.
Terraria - Cheapish RPG with a ton of replay value. Guides are helpful and IMO give the best experience.
The Binding of Isaac: Rebirth - Like the original Zelda but with darker tones. Is much, MUCH harder than the original Zelda. IMO one of the hardest games to play competitively/speedrun well.
All games on this list are offline, cheap and older/not resource intensive.