We stayed 10 days, and got Disney tickets that allowed any 7 days in the 10 day period. I think we actually used 6 days, between the water parks and safari one as well as the 3 main ones (we went to Epcot and Magic Kingdom a few times each - some days did a water park in the morning and a main park for dinner and late evening rides and entertainment).
We also went on a canoe trip down a river about half an hour drive away - with a guide and seeing all the local wildlife, with a picnic and swimming at the end. Which was lovely and very different.
And a day at NASA in Cape Canaveral. Which was fascinating and not long enough for us!
Dd was about 6 when we went. We got an apartment in a complex which had a pool and had an outlet mall across the car park. And we went to a few other malls as well.
We felt at the time dd was too short for most things in Universal so didn't bother going there (but she could do most rides in Disney).
We were there for 4th July and it was hot and humid. Having the apartment was great to go back for breaks and having lots of swims to cool down. Drink lots of water and let kids run through the fountains that are scattered around - they dry out again in minutes but it does cool and refresh them! Even adults need breaks in places like Disney. We also had a renta car which meant we could get out and about when we wanted and weren't restricted to park buses etc.
One thing with Disney was they had maps of all parks in each of them, so we got these and planned the evening before what 1 or 2 things we'd really like to do in tomorrow's park, knowing it might mean long queues, but figuring out where there were food places near those or an ice cream stop just before joining a queue etc. Made it much easier for dd to handle.