Daft question, but have you asked your DD if she wants a theme? Some 18 year olds wouldn't like a theme, she will know her friends. For that number, what about a casino theme? You can usually get roulette and poker sets quite cheaply in the run up to Christmas, would allow them to dress up and you coulddo cocktails and canapes. at the other end of the spectrum you could do a children's tea party with pass the parcel with forfeits, an adult version of pin the tail on donkey (can't think what), party boxes, dress up as school kids.