Seattle is the home of Microsoft, so of course they'll have stuff to show off at PAX Prime this weekend. Specifically, Xbox One games. You'll be able to go hands-on with Ryse: Son of Rome, Killer Instinct, Forza 5, and Battlefield 4. Plus there will be one other game you can play, but they're not saying what exactly yet.
I think I know what it's going to be, and if it's what I think it is, well, I'd recommend you not waste anytime waiting for Ryse. Well even if it's not what I'm thinking it is don't wait for Ryse anyway. It's horrible.
Other titles you can expect to see, but not play, include Kinect Sports Rivals, Dead Rising 3, Project Spark, and Fable Anniversary. There's also going to be stuff with Halo 4 Global Championship if that's your jam. Check out Major Nelson's site for the full details on all the Xbox One happenings.