A space party sells itself on darkness and cardboard, which happen to be the two cheapest materials there are.
One large box, cut with a door and a window, painted or covered in foil, is the whole set. Children take turns "flying" it for a photo or a countdown — the box does the work, not the decorating.
Glow-in-the-dark stickers stuck to a ceiling before the party, or a torch and a dark room, turns five minutes into the quietest, most absorbed part of the afternoon. This is worth doing even for a daytime party — close the curtains.
Grey or silver-painted stones hidden in the garden for a "moon rock hunt" reuses the same mechanic as the dragon hoard and the dinosaur dig — hide something, give a container, let them search. It is the single most reliable activity across every theme on this page.
No. Naming the planets in order is enough context for a party; nobody at a four-year-old's birthday is being tested.
Round crackers as "planets", a fruit skewer as a "rocket" on a stick, and anything silver-wrapped. Naming does the theming.
Part of our guide to Birthday party for one obsession.