Just make sure that, with such short parties, you don't have really long skits or presentations - you want to leave time for conversation.
Themes that have been used on my campus:
- 1001 Arabian Nights
- Mardi Gras
- Parisian Cafe
- Baseball (this one was a dismal failure and that sorority never used it again)
- Jungle
- Casino Night (problem is that both sisters and rushees focused on the games rather than talking about the sorority)
- Down on the Farm
- Theatre
- '50s
PM me if you want details. The trouble is that a lot of these themes won't make sense without decorations...