An opening video. Not too long (about as long as the Valve sequence), it can show off a few features of Garry's Mod like the spawning, building (...gamemodes, if you must ), etc.
But the time limit would make it harder to do stuff, thus we would get better entries. I thought of using little snippets of easy to identify stuff, like building and posing. You could use fly-by shots and those flash transitions. Kind of like the opening for Brawl.