For my 30th I had a Sci-Fi party.  I cleared my living room of all furniture.  Covered the walls and ceiling including doors(making sure they still opened) with black rubbish sacks using blue tack. In the window I covered this also and cut a hole and stuck an earth rise poster in the gap which I got from the Kennedy Space center, and placed a light behind it so it glowed.   On the walls we covered them with fluorescent stars and pictures of space ships and aliens etc.  I made from foil pie trays space ships and UFO's and hung them from the ceiling.  I had Christmas tree lights in the shape of stars running around the top of the walls.   I bought some blow up aliens and this set the scene for the party.   For my cake I mad a Martian landscape by making an ordinary sponge cake and using green butter icing covered it. Then got some ice cream cones and used food coloring to paint them, then cut the ends off to make peaks and used the ring that was left to make craters.  I found lots of other space related food like star shape crisps etc.  Everyone came in fancy dress, and I had sci-fi movie sound track music playing. Great time was had by all.