We are going at Easter this year, well the week before actually, and have gone before at that time. it's crowded but not overbearing and is still an ejoyable relaxing trip.
If you don't want to spend your whole time in the parks, make sure to at least get in Magic Kingdom and MGM. I say this mostly for your kids, and their ages. EPCOT is wonderful and I could spend full day upon full day there, but for younger kids, once you get out of Future World, they will be bored to death.
And Animal Kingdom is great too, but especially if you don't want to spend a whole day at the parks. It's not JUST a big zoo, but is has the qualitites of being a gigantic exotic zoo with some rides. I love the park, but it's true that if you get there early, you can enjoy yourself and be out of there by 1 or 2 in the afternoon.
Quite possibly, if you get park hopper tickets, you could get in Magic Kingdom one day, and half a day at Animal Kingdom and the other half at MGM or EPCOT.
Sea World is a great park. Kraken is an awesome roller coaster, and the shows really are fun to watch at any age.
If you want a few other things to do:
~Downtown Disney - walk around, enjoy the shops, DisneyQuest
~Mini Golf - Fantasia or Winter Summerland