My advice for Magic Kingdom: allow yourself extra time to get into the park because of the necessity of taking either the monorail or the ferry boat from parking to the park itself. If you are there during the summer months, this is the hottest park because of the crowds and all of the concrete and buildings. Be prepared for that, and dress in light fabrics. Take a midday break if you can!
The Mickey ear balloons are my favorite souvenir from Disney World - I've gotten one every time I've gone there since I was a little kid - PINK of course, but just the Mickey ears, no clear balloon around them.