You can't play Watch Dogs, but you can watch Watch Dogs
Ubisoft is the first major player out of the gate to reveal their lineup of content for PAX East next month. On the AAA front, fans can't play it, but they can watch Watch Dogs be played in a never-before-seen demo. Plus there will be an experience where you can see "through the lens of a hacker," and you can get a free t-shirt if your pre-order the game from the booth.
On the playable front, you can get your hands on The Crew, Trials Fusion (but why would you want to do that when the game is out a few days after PAX), Child of Light, Might & Magic Duel of Champions Online Card Game, Ghost Recon Phantoms, and The Mighty Quest for Epic Loot.
If you were to wait for anything here, go with Child of Light. It's such a wonderful little gem.