I downloaded the demo of Boom Boom Rocket, a new Xbox Live Arcade game made by EA, and while I'm not a big fan of EA games, I do have to say that they got this one right.
Essentially Dance Dance Revolution, with buttons instead of a dance pad, the game challenges you to set off fireworks to the beat of music as you slowly fly through a city. The visuals are amazing, but are hard to take in because you're concentrating so hard on the buttons.
This is the perfect party game, easy to pick up and understand, fun for others to watch, and quick enough that everybody gets a turn.
I suggest downloading the demo for some free and fun gameplay.