Hi Akhil, You can consider visiting the following places as part of your solo trip in Dec:
1) Jaipur- Be mesmerized by the pink city and current capital of Rajasthan
2) Jodhpur- Bask in royalty at the Golden city
3) Udaipur- You will fall in love with the innumerous lakes here
4) Jaisalmer- experience life on the desert
5) Nagaland- has the Hornbill festival in Dec
6) Agra - Witness the splendor of the Taj Mahal!