Playing through Deus Ex: HR on the hardest difficulty. I also picked up Prey and Quake 4 for about $3 together at GameStop. After that, I was planning to go through Metro 2033 on the Ranger Hardcore mode, assuming I can get through the other games before Rage and Battlefield 3 come out.
Btw, I don't see why so many people complain about DX:HR being too hard. I'm practically breezing through the game with only the occasional boss fight and giant robot to slow me down. Even then, if you have the right augs and/or equipment you can still blast through them easily enough.