We did a land/sea cruise package last September, and my little ones were 2 and almost 4 then. So, concentrating on the 2-y.o., we had lots of fun but we also paced him well with trips back to the room for mid-afternoon naps and pool time.
Attraction-wise, we only spent half days in most parks, with the exception of MK. So, at MGM we did okay on the Great Movie Ride, Muppetvision and the Little Mermaid show. At Epcot, we didn't see much, but we did have an excellent dinner in the cafe/cantina at the entrance to the world showcase just as the millenium pagaent was happening by. We also had great fun at Typhoon Lagoon for an afternoon.
At MK, modes of transportation rule: riding on anything was fun: the railroad, the People Mover or whatever it's called now. He loved the playground in Toontown, which is where he stayed with Mom while his older brother and I rode Goofy's Barnstormer several times in a row. However, he probably had the most fun on the Buzz Lightyear ride, but he wasn't interested in shooting, he was interested in spinning the car around!
All in all, we had fun and the kids did well. On the other hand, I would caution that I seriously doubt a 2-y.o. or younger could take a long day in the heat unless they nap well in a stroller. Ours didn't, so we went back to our resort frequently and got very tired of loading and unloading strollers on buses. I pretended not to notice that we were spending big $$$ per day on park admission for only spending 3 or 4 hours at a time in parks. I had a much more enjoyable vacation that way