From Daria to Clueless to Gwen Stefani, this great Buzzfeed article has all kinds of great ideas for girls who grew up in the 90's and those who just happen to love the decade. Two of the costumes are technically from the 80's, but they were both from movies girls of the 90's simply adored (The Labyrinth and Beetlejuice), so they still have their place in this article.
While I was always a huge fan of Hocus Pocus and The Craft as a girl, I think I'd much rather go as Log Lady from Twin Peaks because it's hard to come up with a more eccentricly hilarious 90's Halloween costume.