You've got some great advice! We've been twice with DD who will be 5 in March. First time she was almost 4 (1 month away) we went for the day to the MK only, it was dreary and rainy but she had a blast. We had breakfast at the castle. We ended up on only a few rides (Small World, Pirates, Jungle Cruise, Grand Prix and the Carousel (sorry about the spelling!) and spent the rest of the time waiting in line for autographs and she LOVED it! It took all of our patience to not drag her on rides and we kept telling ourselves "this is for her!".
Second time was in September, she was 4 1/2. She loved pin trading (first time we had done it), the cm's are very kind!, getting autographs, swimming in the pool, the night time stories and wake up calls from Mickey. We had her stroller - we bought an umbrella that is larger than most - and used it for either her or the backpack - and a fan that clipped to it -that was a SUPER tip we got from the boards! We went to the pool every day from about 1 - 3 (forget about napping, even though I wanted to! Too much excitiment for a 4 year old to handle!
) Chef Mickey's was super and we were able to skip some waiting on meeting them at the parks. She made a list of "must-do's" before we left.
Oh one other thing that was fun, gave her a disposable camera, she got some great pictures!
DD's pretty risk-adverse but she did try some new rides and had fun. She wore her princess costume to the castle and her minnie costume to Chef Mickey's.
Hunting for Hidden Mickey's is fun, she's still at it.
Waiting in line wasn't a problem as there were no lines anywhere!
Hope your getting some good ideas!