Here's some cute themes that either my chapter has used or that other sororities on campus have used:
SAI Spirit--use red and white for colors, and t-shirts can be cheerleader-inspired, like with a megaphone on the front with the letters. Decorate where you're having Rush with red and white pom-poms, red and white streamers, balloons, and anything else you can think of.
Sisters Wanted--like the Volkswagen ad! Shirts can have Beetles on them. Very cute idea...I saw a sorority that used that and I really love it, because I'm a VW fanatic!!
Oh the Places You'll Go--like the Dr. Seuss book.
Divine Secrets of the SAI Sisterhood--like the movie. We did this for our informal rush. We watched the movie, made silly headdresses like they did, and then have a serious speech about our sisterhood. A very fun, easy to put together idea.
Hope these help. We could use some ideas as well, so feel free to share ideas!