I agree that Yunnan is a great place to visit in December, as is Huangshan. Note, however, that, though neither is Harbin in winter (-22F), they're not exactly warm. If you go up into the mountain areas surrounding Lijiang, you'll need a warm coat (you can actually rent one in Lijiang!). It's rather chilly in the morning though, happily, there are plenty of nice little tea shops you can duck into to warm up. Huangshan is, of course, a mountain ("Yellow Mountain"). I've been there in December. Though, during the day, it's quite comfortable in full sunlight, after dark and at sunrise it's cold.

That said, these are great places to visit (along with Kunmin and Dali) and offer the added bonus of being off-season and not, therefore, too crowded.