We did 5 weeks this year in Orlando (we tend to do at least a month every couple of years)
As others have mentioned :
-Book everything seperately (no need for a package unless you CAN get it cheaper)
-Rental car all the way. Driving in the US and expecially Florida is easy and you will want the flexibility at the parks.
-I am going to disagree with VincentHanna here but Disneyworld is not overrated, the parks (remember there are 4 of them and 2 water parks)are the best by a clear mile in respect of the WHOLE experience. With the 5 weeks we were there, most of it was at one of the 6 Disney parks. I have an 8 year old son and a 4 year old daughter.
-Use the Disney fast pass system like a demon, and book in advance, and ideally all of your 3 early in the day to allow you to do it on the fly.
-Lots of websites with great deails for theme park tickets. We use
https://www.orlandoattractiontickets.co.uk/. They are usually the cheapest (marginally) and they have never let us down.
-Universal Islands of Adventure and Busch Gardens are great for white knuckle rides
-Universal....the original park is over rated. The only reason to go is to experience Harry Potter's Diagon Alley
-Legoland is the poor cousin over there and no better than the Windsor offer. Only go if you can get the 25 quid offer.
-Unless you REALLY want to go, avoid Seaworld. Its nothing like it was. The only reason to go would be if you did the Dolphin experience at Discovery Cove and they bolt Seaworld and Busch Gardens 14 days entry onto that pass with the price
-Don't miss out on Disney Springs. Apart from the shopping there are lots of hidden gems there.
-We never stay at Disney hotels. Villas are generally cheaper, larger, have a private pool and allow you to ear healthily by cooking yourself. Championsgate is a great place to stay. Its like 15 minutes from Disney and 20 from Universal
-Evening shows.....pirates, medieval times.....both brilliant
-Allow a budget for shopping. a lot of outlet shopping there and my missus went crazy and spent like $3000 on bags!
-Take sun block and lots of it....In the summer the heat isnt bareable. Its also especially humid.