We've done luau/Hawaiian theme, circus (the bouncy house was fun and I love cotton candy but some girls are afraid of clowns, so tread lightly here), and movie night. If you get a bunch of paint pens and glass candy containers (cheap at Walmart), you can have a make-your-own Sigma Kappa candy jar. I still have the one my heart sister made for me in 2001!
If you google icebreaker games, you'll find a bunch of different ones that you could play. Just caution all of the girls (especially the actives) to keep it clean. You don't want to frighten the poor new members!