A few ideas:
1. Go as streakers. You wouldn't really be streaking, you'd be acting.
2. Go as drunks. Cost: cheap booze.
3. Go as The Verizon Network. Everyone keep their phones by their ears and follow some poor schlep around town.
4. Go as "Secret Service". Pick one person in your group to be the "VIP" and everyone else wear suits and sunglasses. Gang tackle some guy when he isn't expecting it and rush your "VIP" out the door.
