As pp-ers have said, do not forget sunscreen. Even in the winter, you can get a burn in Florida, especially if you're fair. The sun is also brutally brightm so sunglasses are a must, but bring cheap ones (I lost a $250 pair last time I was there. My toddler kept pulling them off my head so I handed them to my husband and never saw them again). I live only a few hours north of Orlando, and the weather can change without much notice, so be prepared for different weather. Layers are always good. If it does start raining, this is a good thing! Crowds will thin out quickly, especially families with children, and you can enjoy shorter lines. It's also pretty common for rain to not last long, so you might have sun an hour later. Wear something with pockets so you can stick your map and schedule in there and pull it out without messing with a bag, but if you keep your tickets or ID/cards/cash in your pockets be sure nothing will fall out.
If you can use a belt-pack or something along those lines, do it. Some rides won't let you bring a back-pack and you'll have to use lockers. Make sure the camera you bring is pocket-size, and a zip-lock bag to protect it from water (from a ride or the sky) is a good idea. In general, take as little into the parks as you can get away with, but nothing loose that could fly off on a ride, or valuable that you might lose.
If you're staying at a Disney resort, you can charge purchases to your room with your room card, meaning less cash to carry. The Magic Kingdom is dry, but for the others you'll want your driver's license on you if you plan to purchase a drink.
I wasted precious time once by planning to eat breakfast at the Magic Kingdom. Stupid. Eat before you go or bring something, and spend the early hours doing as many rides as possible before the crowds pick up. Then, at lunchtime when you're ready for a break you can enjoy a nice sit-down meal. Any of the non-counter-service restaurants will require reservations, though, so if that's on your agenda book now if you haven't already done so.
Study the maps before you go so you'll spend less time trying to figure out what's where while you're there. That said, take with you also patience and flexibility. When you're stuck in a line, enjoy the general atmosphere, and be ready to change your plans to adjust for what is and isn't crowded, and how you and your traveling partner feel at the moment.
Hope you have a blast!!
