My general advice is to have realistic expectations. My DS was just shy of his 3rd birthday when we went. You absolutely will not and can not see and do everything...If your DD insists on going on small world 4 times in a row rather than go to the last Philharmagic show of the night...hakuna matata - you're on vacation...just go with it. You'll catch whatever you miss (hopefully) next time around.
As far as packing - As prev posters said, don't pack any (or many) toys - only what you may need for plane/car rides and while waiting in line. The best one we brought was a travel size Magna Doodle - light, flat, no issue if it got wet. And a flashlight (from the dollar store) - some rides and park areas can be a little dark and spooky at night, so the flashlight made him feel a little better. I also gave him his own disposable camera - the pictures were hysterical and he felt very important when he took them.
Make sure your stroller is light weight if you bring your own...there is a lot of folding/unfolding involved. I would definitely try to coordinate outfits so that you have long/short sleeves and pants/shorts that can all match together so you can just change part of it if the weather changes. (Did that make sense??)
SIDE BAR - My DS was mid-potty training when we went. In the event that you're doing the same, pack POST-IT Notes (DO NOT LAUGH) to cover the sensor thingy on the auto-flush toilets. I swear by the time I got the paper seat cover on and my son ready to go, the thing flushed 3 times...I had read about someone suggesting that on a prev thread and thought it was crazy, but wished I had the darn things while I was in there.