What space-loving child hasn't enjoyed a moment or two of imagined space travel? A hand-painted mural of a UFO or spaceship cockpit could bring hours of joy to your budding astronaut. Paint a wall or two silver and create a scene that looks like a cockpit with lots of sliders, gauges, knobs and dials. Paint fake windows (or use your real windows) so the space ship has a view of outer space. Use other walls to paint other parts of the ship or paint them indigo with views of planets, galaxies and mysterious objects in the sky.
If your child enjoys stargazing, the walls and even the ceiling could be painted with constellations. Paint a dark background, such as indigo, and copy some of the night sky's constellations on various walls. For an older child, you could make the walls quite detailed with a number of constellations copied from a sky chart. For a younger child, a few simple large constellations such as the Big Dipper or Orion may suffice. Add glow-in-the-dark paint to the stars to make the constellations visible at night. Stick-on glow stars are an option as well. The ceiling could also be painted as a mock-up of the night sky.
A view of Earth from space is quite striking. A view that includes the moon surface, even more so. You can create the look by painting a short moon horizon with a slight arc 1/4 the way up the wall. Paint all areas above it indigo like a night sky. Paint Earth up in the sky. The bottom portion of the wall can be painted with craters, valleys and even a moon buggy or lunar lander, if desired. Continue the part space/part moon surface theme on other walls, or simply paint them as entirely outer space. Flecks of glow paint can create stars, nebula and a view of the Milky Way.
An alien-themed room could be fun especially for a young child. Paint the walls indigo for that deep space look and then add a variety of cartoon UFOs, aliens, rockets and comets. Planets are another option: various walls could be painted with large depictions of Saturn, Jupiter and other planets complete with aliens on a space mission. Glow paint stars are a nice treat, as they are visible even with the lights off. Glow stars on the ceiling are even more exciting.