I must admit I was a bit concerned about the crowds before we went last Easter. In the past we've always gone out of our way to go at quieter times but now we're stuck with going in the school holidays so it was a case of having to lean to live with it. As it turned out, I needn't have worried and we actually found it quite manageable.
We stayed off site in an apartment at Windsor Palms. The resort was busy round the pool during the days but by staying close to the parks it was easy to do them in small chunks because it took no time to get there. Of course, staying off site we weren't able to take advantage of Extra Magic hours which would have made it even easier. Our kids were 12 when we went and were able to stay up until around 11 o'clock without problems by mixing late and early nights, but if yours are a lot younger that might limit you.
First thing is to plan. Make an itinerary of the places you want to go and take into account the extended opening hours and which days of the week are likely to be busiest in each park. We avoided going to the parks for the 4 days of the Easter weekend itself and did our shopping then. By all accounts, those would be the busiest days. The week before Easter and up to the Friday after were also busy but if you can manage 3 weeks there you should be able to fit in 7 - 10 days when it is much quieter.
We found no problems at all getting on everything we wanted at the Magic Kingdom. Sometimes we went first thing in the morning and left at around 11:00, other times we would arrive at around tea time, do a few of the less popular atractions then see Wishes. After Wishes loads of pople leave the park and that is the time to do the big rides. Almost all of them were a walk on. Having said that, one Saturday morning we did Space Mountain 4 times in a row with no wait at all. Another day we did Thunder Mountain 3 times in quick succession at about 5 o'clock in the afternoon. I don't know why these anomalies happen but you have to take advantage when they do !
Animal Kingdom was no problem either. We either went early or late. Mornings before the park gets crowded are good for the animal attractions. When the park is open after dark the animal attractions close at dusk but the rides and some shows keep going. This is great because most people leave when the safari closes so you get all the rides to yourself ! Lost count of the number of times we went on Dinosaur and Triceratops Spin.
Epcot is never much of a problem. Even when it is busy it doesn't seem too crowded because it is so big. Using fastpass you'll get on all the rides without too much difficulty. In the evenings there are often short lines for the rides in Future World anyway as everyone moves to World Showcase. In World Showcase many of the performers are outside so crowds and queues are not an issue.
The only park where we never really got the measure of the crowds was MGM. It always seemed busy and we never did get on Tower Of Terror or Rock'n'Roller Coaster, but by arriving in plently of time for Fantasmic it was no problem getting seats.
We did the water parks for the first time that holiday and they were amongst our favourites. Arrive early to stake out a good spot with some shade and look to leave early in the afternoon before it gets too hot.
The only other park we did was Sea World. It was the busiest I've ever seen it but still Ok because most of the attractions there take place in large outdoor theatres which hold thousands of people. The few rides there are had huge lines in the day but by evening they had gone.
Mealtimes were no problem for us as we were not usually in any of the parks then. When we were we made a point of eating early or late otherwise I'm sure you could spend a lot of time queueing for food. If you want to eat in any of the Disney restaurants then priority seating is a must at busy times.
In summary, We would definately do it again. Make sure you plan. Take advantage of the longer opening hours. Use fastpass.
Hope this helps.