I just remembered a theme that we used for our informal Spring Rush during my first year in DZ. It was a luau theme, and all of the sisters dressed up in Hawaiian clothing... some were really outrageous with loud Hawaiian shirts, grass skirts, and sunglasses, sporting cameras around their necks like tourists and others wore really pretty casual sundresses. The only thing that we wore in common were the leis around our necks. I liked it because our choice of clothing gave rushees an insight into our individual personalities, and helped to show them that we weren't just a cookie-cutter type of chapter.
We did a limbo contest, had fruity "mocktails" for refreshments, and just mixed and mingled with the rushees. It was a lot of fun.