A few other ideas to make your get-together interactive:
1). Have a UFO-themed movie like E.T or a cartoon playing on TV (Lilo and Stich), or else some background music like "The Purple People Eater' by Sheb Whooly or Will Smith's "Men in Black".
2) Search for clues to the alien ship using a list of clues to follow.
3) Make crafts out of paper plates. Have a table set up with things to decorate the outside of the plates (Pens, Glitter, glue, stickers) and then glue the plates facing each other to make a flying saucer. Make sure and use a heavy glue that binds paper quickly. They can take these items home as gifts.
4) Have the group come up with a Martian language.
5) Make a cardboard cutout with holes in it, put the point value on top of each hole, and then have the kids try and fly mini Frisbees though them.
6) Many of the dollar stores carry the bobbly antennae head bands. These would make great "welcome to the get-together" gifts and also be fun for the group pictures.