"Extra Toggles" in Lego Star Wars. Useless characters (like Gonk Droids) that are all really awesome
Hacking the Swinekeeper game, hell, Swinekeeper itself in System Shock 2. Seriously, try hacking the mini-game thing :D.
Also, try shooting a basketball through the basketball hoop :D.
Ahh, yes. I love the included minigames in both System Shock 1 and 2. Nothing like catching your breath in a safe room by playing some Minesweeper with pigs.