We took our 4yodaughter In March, and also when she was 2 1/2. The great thing about Disney is there is not much they CAN'T do. On the 2 1/2 trip we also went to Busch Gardens, but we were disappointed in the amount of attractions for our daughter. Even in kiddie land she was too small to ride most things alone, and most didn't allow adult riders on those rides. She rode the carousel there and I think that's it. In March, she rode everything we did except Space Mtn and RockNRoller Coaster. She even rode Tower of Terror and Test Track (3X). Here's my recommendation:
Do everything in Fantasyland and Toontown, watch the parades. My daughter also really liked Pirates and Tiki Birds and all those kiddie shows at MGM. Get the autograph books & meet as many characters as you can. Go to the character tent at Toontown (MK) and you will see a lot of favorites. There are 3 lines -- one for Hundred Acre Wood characters (Pooh & friends); one for Disney favorites (Minnie, Donald, & Goofy when we were there); and one for storybook friends (Cinderella & her mice when we were there). All the parks have character greeting areas, so you can look for your favorites.
Definitely do a character meal or two. We have done the char. breakfast at O'hana (Polynesian resort) and this trip we did the Ice Cream Social at Epcot (at the Garden Grill Restaurant in the Land). If you do a search you will find info on this, or read my trip report posted on the trip reports board. Both of these events had great food and superb character interaction.
Enjoy!