Audiosurf - Surf your music. Some people really love this. I play very casually.
Bastion - Fun and beautiful Action RPG.
Drox Operative - Fun little and mysteriously addicting space RPG game - would go for EV Nova or Transcendence if you haven't tried those yet.
Dwarf Fortress - Yes. Don't tell your parents about whatever you do to the elves, though.
Faster Than Light - Amazing.
Gratuitous Space Battles - Design ships, make your battle plan, watch results.
Mount & Blade - Too much violence I guess?
OpenTTD - Transport Tycoon Deluxe, oh yeah.
Port Royale 3 - Trading (and more) on the Carribean.
Starbound - Terraria-like Sandbox.
Startopia - Great fun strategy game. One of my all-time favorites.
Street Hacker - Simple Hacking (Console based).
Space Rangers - Great turn-based action in space.
SPAZ - Fun Top-down space action
Spore - Great fun (even without turning out to be the best game in the world, spore is very fun).
Prospector - Hard roguelike in space.
Rollercoaster Tycoon - Everyone must have played this at least once.
Teacher Story - Webgame but I found it hilarious.
The Sims Medieval - Actually fun, deserves a try.
Sim Theme Hospital - Old, but classic.
Uplink - Simple Hacking (GUI based).
X3 Terran Conflict or X3 Albion Prelude - 4X Space arcadey-Sim.
And you can always waste a lot of time on
kongregate anyway.
Or emulators.
If everything fails, do as Naxza said.
Reading is great.
Also series and animes if you're into that.
And don't get cave addiction.