I usually try to go mountain biking on Christmas. It was much easier to do when I lived on the west coast (friendlier weather, fewer family obligations). This year, I wasn't near any good single track, and had a bunch of family obligations, but I still managed to get out for a bit. I grew up near Acadia National Park. There isn't any 'mountain biking' there but there are 'carriage roads'. I wouldn't say the carriage roads are fun, if you're expecting single track. They're dirt roads. Unless they're covered in snow... Oh man. Definitely challenging. And fun. And totally deserted. Which is perfect if you need a little break from all of the holiday cheer.