My son was 3 for our last trip (last year) and LOVED the Speedway. Like, I went on it 8 or 9 times in a row. Once he figured out he wasn't actually driving the car he was annoyed. The same happened at Disney Jr--we went in, all excited to see Mickey's Clubhouse and his first words were "Mama, that's not a the real clubhouse, you can't even go inside it."
We had great success with Dumbo, Pooh (he liked it all but the thunder/lightning part), Teacups, and Peter Pan. Buzz was a too hard from him at 3 and he didn't really understand what he was doing. People Mover also thrilled him, as did the Monorail. At HS he liked the HISTKs play area, but it was stressful and hot as a parent--crowded and difficult to keep up with him in the small spaces. This time he'll be 4 and I feel less compelled to go with him, but last time he had just turned 3 and had more than once found himself stuck in a play structure, so I went with him. At Epcot the Living Seas interested him for a quite awhile as did Turtle Talk, but the biggest hit were the jumping water fountains. We were there after dinner (6ish) one night and spend 90 minutes just running under them. Sometimes alone, sometimes wiht other kids, but it was really empty and he loved it.
THe pool was probably the most requested activity though, daily. Even now he only talks about the pool.