i know this is kinda stereotypical, but last year we had a toga dance. it was SUPER fun b/c everyone wore different kind of sheets...and it was interesting to see what ppl had chosen to hold them up with...this year our theme dance was a pj dance "alpha gam pajama jam" which was really cute...esp. since that night we were under a tornado watch. we were having so much fun, though, that no one noticed.
you may, however, want to impliment a dress code...
monica