Ever Play Garry’s Mod on Steam? Hands down that is my Favorite Steam title. From Garry’s Mod.com: Garry’s Mod is a Sandbox Game based around the idea of building. Unlike most other games there aren’t any objectives – you can’t lose and you definitely can’t win. That is the beauty of it. Building is quite popular, I’ve always enjoyed the many new Maps and NPCs (Non-Player-Characters) that the Mod community churns out on an almost daily basis. More fun than should be allowed by law.
I could go on forever. My son and I play together on occasion.