HEY-LO!
I'm Patti. I'm 17 (18 in 15 days!!!) and I'm from New Hampshire.
It'll be hot while you're there (no brainer) so definately bring water. If you have one, bring a water bottle that you own (like a Nalgene or a Sigg or somesuch) and keep re-filling it with water from the fountains, or at counter service restaurants just ask for a cup of ice water (it's generally free. They can't technically charge you for water) and fill your water bottle. This will save you the $1.50/$2.00 per bottle of Dasani and leave you with extra cash for spending on souvenirs.
As a PP mentioned, definately check out Kaki Gori at Japan!!! I get one every trip, no matter the weather! It's just shaved ice, but it comes in some neat flavours. I love getting half mango and half green tea!
Another great snack is frozen tea (if you like tea...even then, it doesn't much taste like tea). You can find this at two parks-- Animal Kingdom and EPCOT. At AK, it's found at the Royal Anandapur Tea Company in Asia (by Yak and Yeti) and at EPCOT it's the Joy of Tea stand by China. It's a great way to cool down.
Take full advantage of your hotel pool and of Fastpass and Extra Magic Hours, too!
