Make movies or take pictures together (tie yourself to a railroad track, pose with statues, race wheelchairs in the hospital, etc.).
Invite everyone to bring a white t-shirt to decorate. Buy some dye, string, and set to work tie-dying the shirts. A different, colorful design might be just the thing to dress up that old shirt.
Have an ice cream sculpting contest. Each couple gets a block of ice cream, spoons, knives, and toppings. This is a messy activity, but lots of fun.
Play a mud football game after a rainstorm. Change clothes and have some hot chocolate.