The top 3 things in my bag:
1) SHOUT WIPES
These little pre-moistioned towelettes are pretty amazing. They are great for removing any kind of stains without the aid of water. You may have a day where you have to eat lunch between parties with no time to change or you may accidentally stain your pants sitting in a lawn chair under a tent. Accidents happen. That's why they made Shout Wipes
2) BAND AIDS
I know several blister preventatives have been suggested but band aids are great in the event you get a blister, and also for other cuts and scrapes you might encounter during rush. Plus, you'll be a new best friend to the girls in your rush group who DO get blisters and you're there with a handy bandaid :-) Sure, your Rho Chi might carry them but buy a $1 pack from the travel aisle at Target for yourself, you may appreciate it later.
3) iPod
If you have an iPod or similiar MP3 player, take it if its not too bulky or heavy. Rush is stressful and I personally found it nice to be able to pop in my earbuds between parties. It helped me relax and also drowned out the notorious "tent talk" I tried to avoid hearing.
I'm looking foward to your rush story! Best of luck!