I decided to go with the theme “Swarms.”  Here is a mockup of what the final game might look like:

I already have the people and aliens running around.  They don’t exactly “swarm” in any coordinated manner, but I’m not going to worry too much about it.

After lunch I’ll impliment the player’s ship, tractor beam, tossing aliens, and alien ships.  I’m just going to hack this together as quickly as I can.