Flags. Some might find it offensive to alter the American flag, but maybe you can find a way to do it respectfully. Those Vassar banners sold in the college store would be unique, if you're willing to spend that kind of money.
Balloons. Preferably deflated and made of foil. They probably sell them at the dollar store.
Only duct tape. May seem like a good idea at the time (it stays in place), but think about the painful removal.
Garbage bags. Sure it's cheap and wouldn't require much alteration, but that Villard Room is going to get hot and you'll want something that will breathe.
You can also do a full out costume. Wear plastic flowers and plants and be Adam or Eve. Or cover yourself in wrapping paper and a bow and be a "present." Remember, keep it classy. Scantily clad students have been thrown out in years past.