Our favorite party ever was a Space Party, which all the kids referred to as "A Glow-in-the-Dark Party," once they arrived.
We decorated the house with glow-in-the-dark stars, and set up two large fluorescent black lights, which we found at a novelty store. We told the kids to all wear white cotton so they would glow as well. The kids had so much fun running around in the dark to some upbeat music, we really didn't need to organize games. They just had fun on their own.
The cake was a retro-looking rocket ship (something you would have seen on Rolie Polie Olie), that we made from a cake baked in a football shaped pan. We just added fins, and a window made out of Jell-o.