Cut out simple stars and planets from construction paper, glue them to the end of streamers and hang from the ceiling to create a 'spacey' atmosphere.
Dim the lights and light little tealight candles around the room when it's time to present the cosmic birthday cake.
Sidewalk chalk can be used to create the blast-off countdown; starting with '10' at the end of the walk, and going back to '0' closest to the front door.
Clear fishing line can be used to suspend objects from the ceiling to create a 'weightless' atmosphere - like that of the moon.
Guests can arrive dressed up as astronauts or aliens, or white, disposable coveralls can be purchased (in smallest size) and decorated with duct tape, kids' names and stickers for guests to wear upon arrival.
Purchase space or alien-themed paper plates and cups at a party store.
A large tent can be set up in a darkened room, and guests can be given flashlights to view glow in the dark stickers stuck to the roof of the tent.
Fun For All
Hang a length of Kraft paper along one wall and let kids paint/draw their own martians.
Find a large kitchen appliance-sized box and let kids decorate with glitter, foil wrap and paint to create their own rocket or space ship.
Play a planet quiz game, where guests (ages 6 and up, probably) are asked planet-related questions and awarded points for correct answers.
For a warm-weather party, guests can practice 'astronaut training camp' in the backyard; featuring an obstacle course, or other activities.