Anonymous wrote:Volcanoes in Hawaii
Los Padres / Big Sur in CA
John Day Fossil Beds in OR
Being slightly nit-picky, but Los Padres is a National Forest (not Park) and Big Sur is a California State Park.
I'm not really sure if it really matters, esp. as OP's question was vague. Lots of places in the US are quite "possible" to go to in the winter, regardless of their status as a national park.