What about doing a Hollywood theme? You could wear cocktail or formal dresses, have a "red carpet," hang gold stars from the ceiling, and drink sparkling cider from plastic (or real, if you have the money) champagne glasses. I've considered using the theme for rush, and I thought it would be cute to cover the walls in large gold star cut-outs, each with a sister's name--kinda like the Hollywood Walk-of-Fame.
Or what about an Angels theme? You could dress in white (white dresses, maybe?) and decorate with white, glittery "clouds." One o' my sisters gave me the idea--she thought it would be neat to wear feathery angel wings (you could easily cover cardboard wings with feathers found in craft stores and spraypaint them glittery). Her slogan was something like, "A down-to-earth sorority that'll put you on Cloud-9."
Then there's always the usual luau (consider an outdoor setting with lit tiki torches--it goes over really well!), Mardi Gras, slumber party (wear PJs and decorate with pillows and blankets), and western. You could use any of these themes and make 'em as simple or as elaborate as you wanted.
Hope I helped you!
