Here's an entity-specific contest that could prove quite exciting.
You're to design something, anything at all, that involves
lasers (using env_laser or env_beam entities). The machine you
encounter at the beginning of HL comes to mind. But don't
limit yourself to big spinning gadgets. It can be a gun, a security
system, a teleporter, some kind of power system. Anything. You
can 'script' it so it does a sequence of events when triggered.
You can use other entities to add sound, light, sprites, gibs, etc. Of course the more exciting and original your 'thing' is, the better your chance of winning.