At that age, organized games and get-to-know-you activities just aren't "cool". Your best bet is to get a song list from your stepson, burn some CDs, and crank up the stereo.
Have a table full of munchies (fruits, veggies, maybe candy and chips if you do junk food).
If you don't have a problem with squirt guns, stock up on some cheap ones and hand them out. They can double as party favors. Maybe fill up a few buckets full of water balloons, too. Divide the kids into two teams, and have a water war.