Hi, you can go for any of the following! 1.) Brahmatal trek.
2.) Chadar Trek.
3.) Goecha La Trek.
4.) Kedarkantha Trek.
5.) Kuari Pass Trek.
6.) Har ki Dun.
7.) Nag Tibba Winter Trek.
8.) Panwali Kantha Trek.
December is a great time, and you'll find snow in all these places :)