WalMart has really cute Disney princess rain ponchos right now for <$5/each.
Even though we bring rain ponchos, I also bring an umbrella, too. It does double-duty for rain protection and providing shade when it's sunny. It really came in handy when my mom and I had to wait over an hour in the blistering sun to meet Tink at EPCOT.
Band-Aid makes a rub relief blister block that works wonders for blister prevention/relief, but a stick of Body Glide works just as well too.
I have a little cosmetics bag that I pack pharmacy-type stuff in: Pepto-Bismol tablets, Lactaid (since I'm lactose intolerant), ibuprofen, Claritin, Plackers & gumpicks, eye drops, Midol, fem. hygiene products, and hand sanitizer.
A travel pack of Wet Ones always comes in handy. I use them to clean up ice cream drips, wipe off tables or handles on the monorail, and even to clean my hands after I reapply sunscreen mid-day. And this may be TMI, but if it's been especially sweltering and I feel yucky, I'll go to the bathroom and swipe myself down with a Wet One to get all the sweat off. It's quite refreshing, actually.