The breakfast club - everyone come in pjs, robes, fuzzy slippers, etc.
The zoo - get animal ears and noses or use face paints. Use black crepe paper to create cages.
Rhinestone cowboys - western, cowboys, bandanas, hay bales, cut western town storefronts out of cardboard.
Jungle - make palm trees out of green and brown posterboard, wear pith helmets and khaki clothes, could also do a survivor theme with tiki torches.
Brady Bunch or Gilligan's Island - have each person dress as one member of the group
Shipwreck - dress as pirates or shipwrecked people, put a book under one leg of each desk and table so that it looks like the room has been shipwrecked.
Out of this world - dress like aliens or the Jetsons and decorate with tinfoil and flashing xmas lights.
Broadway - Top hats and bow ties, use black butcher paper to make a NYC skyline.
Check out
www.orientaltrading.com for cheap decorating and costume items.