My daughter is 4 and has already been to US/IOA three times (not Disney yet, believe it or not). She is freakishly tall for her age, and has ridden most everything except the real big rides (i.e. the coasters, Ripsaw Falls, Dr. Doom, etc). She's a real rider - she gets very upset when we say she's not big enough to get on any of those.
She adoresthe parks. Here's a bit of a breakdown:
IOA
Loves Cat-in-the-Hat, Carro-Seuss-el, One Fish Two Fish, and the High-in-the-sky Trolley at Suess Landing - plus she would play all day in the little interactive water area there.
Has been on Spiderman - did not love it. Probably too intense for her. Enjoyed Jurassic Park very much until she bumped her nose on the (padded) lap bar on her third ride. Wouldn't go again after that. But really dug Pteradon Flyers, had to pry her out of the Discovery Center, and again would stay all day in the Jurassic Park play area, especially the water-gun fights.
She has probably been on the Hippogriff 20 times. Her favorite coaster, and in my opinion probably the best "kiddie" coaster around.
She also insists on multiple visits to the Mystic Fountain every trip. Oh my she loves that thing. We have so many videos of her talking to it. One of the overlooked gems of the park.
Has also been on Bluto's Raft ride - liked it OK (not as much as me), but not really enthusiastic. A bit surprising as she loves getting (and seeing other people get) wet.
Universal Studios
Loves meeting the characters. Lost her mind meeting Spongebob. Really enjoyed (more than me) Shrek (demanded to ride it again and again). Also LOVES the Simpson ride - something I thought would be way too intense for her. She wants us to dress up as the Simpsons for Halloween this year.
Was scared by Jaws. Liked Men In Black (mommy's favorite ride) until the giant bug at the end "ate" us - wouldn't go on it again. Loves the Woody Woodpecker coaster - a real fun ride. Always has to see the Barney show at least twice (sigh).
Was scared to try out E.T., but gave it a shot. Then had to ride it again and again. And now it's her favorite movie (she'll hold my hand when E.T. "dies" and tells me not to worry - he'll come back to life). Is also happy to run around in the Curious George play areas (big splash section, indoor ball pit, etc.).
So there's actually a lot to do for a tall, adventurous little person to do. It certainly filled our days (we usually stay for three or four nights at one of their hotels, so there's also a lot of pool time), and this doesn't include any shows other than Barney.
Right now her regular routine with her dolls is having them dress up so they can move into the "elephant hotel" (the Royal Pacific). She does this so much I think we may have to go back again over Christmas.