We did a theme once, "Shoot for the Stars, We'll give you the Moon" and it was a big hit. I think this one is kind of common for Gamma Phi, but it could be really neat if done well.
Also if you have a local mascot you could set up your room around that. We always had a carousel horse room (our local mascot) and explained how we chose that mascot (We are all different and unique, but moving toward the same goals and with the same purpose.) That always seemed to be popular.
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“All his life he tried to be a good person. Many times, however, he failed.
For after all, he was only human. He wasn't a dog.”
― Charles M. Schultz
Warning: The above post may be dripping in sarcasm and full of smartassedness.