I’ve never done formal recruitment, so I might not be completely in the know concerning rounds and themes for each, but here are some suggestions anyway.
Around the World/Across the Country – This keeps things wide open in terms of what you’d like to do.
Seasons – Have each round be a different season. Serve food that goes with each one. For winter, you could have hot chocolate and cookies. For spring and summer, you could have similar food, with fruit, lemonade, and possibly a BBQ. And for fall, you could have pies and cider. Decorate and dress according to the season.
Or base your themes around your sorority! Think of all the different things you could use: colors, jewels, crests, etc. My chapter only does COB, but we’ve started to move away from elaborate themes and focus more on getting to know the PNMs. One of our themes is simply Green and Gold, and everything will be decorated in these colors. The sisters will wear these colors. Food will be these colors (Hi-C, apple juice, green and yellow candy, veggie dip with cucumbers, celery, and green and yellow peppers, etc.)
Be creative without breaking your budget... or scaring away PNMs!