Hi,
I have been to Florida loads of times, so think I could give you a bit of advice, Im actually due to fly their in 3 weeks!.
Universal is great, you don't just have the Harry Potter at Universal, there are many other rides and attractions, there are going to be 2 new rides next year, one in Studios and in Islands of Adventure. Universal. It will take you probably a day each, full on to get round both parks. We usually take 3-4 days to do Universal. The waterparks you mention, do you know which one you would like to do?
There is Aquatica, Blizzard Beach, Typoon Lagon, the Legoland Waterpark and the New Universal one might be open by the time you go next year. There is lots to do.
If you want to do calmer in the 2nd week, I could suggest Clearwater/St Petersburg which is the west coast, but I must warn you it's where people go to retire so not really much to do.
Miami is a little bit further to travel (aprox 3 hours from Orlando) but South Beach is busy or you can stay in Fort Lauderdale (again Retirement place) a bit north from Miami.
My Husband and I did Miami for 4 nights which was great but busy, We also stayed in St Petersburg for 3 nights, but this was not our cup of tea, again very quite at night.
You could always spend a day at the beach if you want a bit calmer time, Coco beach/Daytona is an hours drive from Orlando.
Hope this has helped