As a former resident of Orlando, I can confidently tell you the best time to go is when school is in session. For the really warm weather, go in Sept. or Oct. Little or no lines (except for Magic Kingdom, it's always crowded no matter what time of the year), nice weather, maybe a little humid, but not bad. Definitely nice for swimming. Next best for small crowds is mid-January to early-mid March, before schools let out for Spring break. However, it can get cold during this time, especially at night. Worst time? Summer. Too hot, too crowded. Next worst is the week before Christmas to just after New Year's. Now, as a former Disney cast member (I'm still in therapy to rid myself of those memories), a few tips. Bring your own snacks and drinks. Yes, it's expensive to get in (I think it's up around $70 for a 1-Day, 1-Park pass, with Park Hoppers more expensive, but well worth the price if you are going to be there for a few days), but that is not where the Mouse makes his money. It's in concessions and merchandise. For example, that bottle of water you can get at the corner convenience store for .99-$1.25? $3.95 to $4.95 in the park. If it's going to be hot, buy some water off property or bring some containers that you can stick in the freezer. Freeze the water. It will melt as the day goes on, and you can refill them from the drinking fountains. Eat before 11A or after 2P, that's when everyone else will still be walking around. In EPCOT, the best place to eat is in the Living With The Land attraction, in Future World, or front part of the park. They have a plethora of little snack bars, from BBQ to burgers to sandwiches, all reasonably priced, at least for Disney. Also, if you go to EPCOT, Future World section opens at 9A, two hours before the World Showcase. When it hits 11A, go to the showcase. Everyone will still be trying to ride Test Track and Mission Space, and you'll have run of the back part. Later in the afternoon, head back to the front, as everyone will then be working their way towards the back. You can always take the ferries from the far side of the lagoon to the near side, saving some walking. Speaking of walking, WEAR COMFORTABLE SHOES! Please don't be like those, for lack of a better word, idiots who would wear high heels because they want to look good. By 5P, you'll wish you had. Also, bring backpacks that you can carry clothing in, or rent a locker near the front. Dress in layers. It may be cold in the morning, but it can warm up by afternoon, then cool down again at night. Also, stay for the end of night activities, whichever park you go to. Illuminations at EPCOT is awesome, but don't leave too early. There's a bit of a lull when folks think it's over, only to get all the way to Spaceship Earth and realize it's not. If you can afford the time, find a place to park yourself around the lagoon, preferably on a bench away from trees, around 7P. If you wait until 8P, you've waited too long.
For Animal Kingdom, it's only open during daylight hours, so that's a good one to go to, then head over to another park for the evening (there’s shuttle buses that take you everywhere. The Monorail only runs between Magic Kingdom and EPCOT. And they are free of charge to use.) If it's warm, ride the rapids, but expect to get soaked.
Disney-MGM, or Disney Hollywood Studios, as they are now called, has some of the best rides. Tower of Terror and Rock 'n Rollercoaster are a blast. The shows (Beauty and the beast, Little Mermaid) are good, also. They also have a closing show called Fantasmic, see it if you can.
As far as the Magic Kingdom, it's a crapshoot. As I said earlier, it's always crowded, no matter what time of the year. Rides are dated, but the Haunted Mansion is still a guilty pleasure, as are the rides in FantasyLand, like Snow White and Peter Pan.
If it's crowded, use the FastPass feature. it allows you to get a ticket that allows you to get on the rides faster, if you return between the times specified on the ticket. That being said, don't get one for Test Track at 11A that says come back at 12:15P, and then go all the way to the back of the park, and still expect to have the time to enjoy your time and still make it back. It ain't gonna happen. Also, one of the best times to ride TestTrack, and I would assume Mission: SPACE (been 4 years since I moved away from O-Town) is during Illuminations. Everyone will be oohing and aahing at the fireworks.
I would recommend against buying lots of merchandise because it's just too expensive. However, if you do, go one of two routes. Either wait until the end of the day, after all the festivities (the shops will stay open as long as someone's spending money). Bad side of this is that's when everyone else shops. (That's why the parks are set up they way they are. When you walk in, where are all the camera and film shops located? On your right, as most people are right handed or tend to naturally go that way. Think about it. What side of the road do we drive on? Exactly. When you leave, where are all the big stores located? Yep, on your right.) Best thing to do, if you have to shop while at the parks, is to take advantage of parcel pickup. It's a free service that all the park stores offer that allows you to shop. The store will then send your package to a designated area near the front of the park, where you can pick up you packages at the end of the night as you are leaving.. Again, it's free, so take advantage of it.
I'm sure I've left a boatload of info out. The books mentioned in other posts are good. I would also recommend mapping out your day BEFORE you go to the park, like the night before. Know where you want to go, and concentrate on those areas. But be flexible enough to change plans if needed.
If you have any questions or concerns while you are there, speak with a cast member. If they can't help you, they will direct you to Guest Services, who will do what they can to rectify the situation.
Hope this helps without overwhelming you. As I said, I've been away for four years, so some things have no doubt changed, but the tips I've provided are pretty much fail-safe. My wife and I are going in a couple of weeks when we visit her folks in Ocala, and mine in Sarasota. I know, SPRING BREAK, not the best time to go, but it's the only time we have, as both of us are taking classes at UNA. I'll let you know if these tips still apply, but I'm confident that they will.
