When it comes to applications on tablets, developers really have to think outside the box in order to create the next addictive thing. Right now, Microsoft Research, Intel, and the University of Washington are working on the Portico, which allows the user to interact with the world outside of the tablet.
You can watch a video after the jump if you want to see a demonstration, and I highly suggest that you will. You will notice how two simple webcams can allow a stencil of a spaceship to blast asteroids on the screen. In addition to this game, Portico can also allow the player to roll a real soccer ball the size of a golf bar in order to try and get it past a virtual goalie.