I haven't got a clue what stuff is like where you live, but here are some good ideas form late teens/twenties parties I've been to.
House party, possibly themed. Good themes are playboy, masquarade, pirate, 80s, sci fi.
Maybe hire a hall if your parents aren't up for the house bit. If you do that get a (good)band/ DJ.
Visit a museum/gallery/ have agirly day out with close friends.
Dinner party, see if you have any talented friends who can host because might be a bit stressful for you otherwise.
Spoof kids party, balloons, cake, games, get everyone to dress like they are four.
And remember it's not about what you do but who you're with.
