We took a week's vacation in the area in early January. It was cold in Tampa and Sarasota part of the week and then it warmed up so you may luck out. I would suggest driving down as far as you can to Marco Island/Everglades and staying there...it was 80 degrees in Jan. and inland areas like Lely Resort area & the Everglades will be 10 degrees warmer (golf, pool, tennis)
There is a lot to do besides the beach in SW Florida, but Busch Gardens to Marco/Everglades is 3 hours apart, which is why I suggested you opt out of Clearwater - you can always drive to the beach there from Busch Gardens area. Caldesi Island would be an exciting day trip if the weather is warm, its near Clearwater, rent a boat or find a tour boat or guide.
We did the Myakka River State Park, Marie Selby Botanical Gardens, Crowder nursery (plants) and those were great for a sunny but cool windy day. Beach at Siesta was really cold and windy. Marco was warm - we spent the whole time poolside at the Marriott and going to eat at Capri Fish House, Snook Inn etc. except for one day when we did a manatee tour from Port of the Islands.
http://www.see-manatees.com/ (see my TripAdvisor review under TropicBird from 2013.)
Everglades City offers a National Park boat tour of the 10,000 Islands area and you can also do an airboat ride or rent canoes if you are into nature stuff. There are boat tours to Anaclote Key out of Tarpon Springs (get some Greek food for lunch) Egmont Key out of St. Pete and Fort DeSoto Island (which has a really great state park beach.) We have stayed at Captiva, beautiful water and terrific shelling, and Boca Grande on Gasparilla Island (a rich man's fishing island with a lighthouse, a deserted beach and small golf-cart oriented downtown.)
Someday I plan to get to Cayo Costa, it is a state park accessible only by boat...like Sanibel/Captiva but less people.