Don't bother with the umbrella, seriously it will break. Umbrellas also don't help because Florida rain always seems to come in from the side. Take a poncho instead, or do what I do, just enjoy it.
It WILL be hot. Don't pack jeans, it'll be a waste of space.
Pack shorts sleeves and tanks, also don't bother with a hoodie. Pack your most comfortable and practical shoes. You'll most likely one athletic shoes, but also pack flip flops for the pool/waterparks.
You'll need rubber bands for your hair, even if you usually wear it down. Honestly, you'll want to put it in a ponytail about 5 seconds after stepping out of your room in the morning. And don't bother with a flat iron, it won't look good with Florida humidity anyways.
You might want to bring extra shampoo and conditioner.
I wouldn't bother bringing your iPod to the parks because it'd be easy to lose, and you'll really be too busy to use it. You may want it for the plane.
Also for the plane (I assume you're flying) -- bring something to do! I'm always stir-crazy on planes, I need a magazine or a book or something to read. You may also want a blanket, it can get chilly on some planes.
Have fun!