Hi there,
Tampa is on the West side of Florida, and has a very nice theme park called Busch Gardens. Apart from rides for all ages, and rollercoasters, there are lots of animals to see, including white tigers, which are gorgeous. There are usually some shows to see as well.
buschgardens.com/bgt/
We have been quite a few times, and our boys really enjoyed it. It is quite hard trying to do everything in one day though. There is a skyway ride like a cable car that goes from one end of the park to the other, passing over African plains with animals like antelope, zebra, giraffes etc.
We like to eat lunch in the Crown Colony restaurant/ cafe, where you can sit outside with a fabulous view of the Serengeti Plains and animals. Our boys said they felt like they were in Africa, not America.
It is a very nice park to walk round. Our boys also like the gorillas, elephants and giant tortoises, and watching hippos swimming underwater. There is a lot to see in one day.
buschgardens.com/bgt/Explore/Restaurants.aspx
It takes about an hour to drive from Tampa to Walt Disney World and other theme parks and attractions near Orlando. There are 4 Disney Parks - Animal Kingdom, Magic Kingdom, Epcot and Hollywood Studios. Disney also have two themed water parks called Blizzard Beach and Typhoon Lagoon. You really need one day in each park.
SeaWorld is a lovely park to visit, with lots of different shows during the day, lots of birds and marine mammals/ fish and our boys loved watching the polar bears swimming from the underwater viewing gallery. There is a waterpark nearby called Aquatica, and also Discovery Cove where you can spend a day swimming, relaxing, snorkelling and pay extra for a dolphin experience.( expensive ).
The other parks in Orlando are Universal Studios and Islands of Adventures, which are next door to each other, and have lots of good rides and attractions for all ages.
Sorry I can't help with Tampa more, but we have only been there as a trip from Disneyworld to Busch Gardens. People on the Trip Advisor Tampa forum can give you lots of advice though, as they know this area very well. There are also TA forums for WDW and Orlando.
Weather in December can vary - it can be pleasantly warm during the day, so ok for shorts and T shirts, but a bit chilly in the evenings/ night. It could also be quite cold and wet, so pack for both weathers. Last December they had very disappointing weather in Orlando, with quite a lot of rain, and not very warm on some days.
Other years can be much better.