Each child can take home a mask when the party is over. The kids can get very creative dreaming up their own vision of aliens encourage them to really use their imaginations. Call a party entertainer to play an alien at your space station. The kids will love it. Or, rent a moonwalk if you have room in the yard and the time of year is right.
Face painting is another fun activity.
You can hire a professional, local teenager, or older sibling to turn everyone into aliens. You can do it yourself if you have some helping hands. Keep it whimsical, colorful and simple. This is crucial otherwise mayhem may set in. In fact, you can adapt almost any of the usual games to the theme.