Sometimes when I don't know what to play I go back to the classic games. F.E.A.R. is a perfect FPS, Splinter Cell Chaos Theory is probably the best tactical stealth game ever made, Crysis never gets old, ever play Arma 2? It's a very realistic military sim you can play for hours. Hmmm, lets see...
Every try the new Medal Of Honor? Underrated game, the multiplayer is quite fun and realistic.