December fishing in the park can be pretty tough but very rewarding. Fishing any of the tailwaters in East TN can be excellent year 'round. Closest to Townsend - in order of distance - would be the Clinch, then the Hiwassee (not so good in the winter), then the Caney (one of my very best days there came at the end of November a couple years ago) or the Holston, then the Watauga, then the South Holston. I have yet to fish the Holston but I hear great things about it most of the year. The Watuaga and the South Holston are terrific year 'round fisheries. If you've got the time and the cold weather/water temperament, I'd plan at least a day or three on a couple of the area tailwaters. Personally, if I had to pick just one, it's probably be the Caney Fork.
Approximate travel times from Townsend:
Clinch River -- 1 hr 15 min.
Hiwassee River -- 1 hr 30 min.
Caney Fork River -- 1 hr 45 min.
Holston River -- 1 hr 45 min. (?)
Watauga River -- 2 hrs.
South Holston River -- 2 hrs. 15 min.