Having no idea what you're after in terms of experience or where you're starting from:

These are my go-to places with good weather in December that are easy for a solo traveler:
Puerto Vallarta, Cabo San Lucas or La Paz, Mexico; Oahu or Maui, Hawaii; Negril, Jamaica.

Cities of Europe are great solo and would be decked out for the holidays. There's a nonstop Paris/Seattle now that is calling my name.

Where I would love to go but have not been yet: Holbox, Roatan, Belize, Kauai, or Buenos Aires.

Iceland would also be cool, especially if you are interested in the northern lights. Days are short so you would have to have a strong interest in northern lights and/or cityscape photography.

There are some amazing parks in Patagonia.

Right, if overseas is an option...

New Years (Hogmanay) in Edinburgh - https://www.edinburghshogmanay.com/

Up-Helly-Aa in Shetland (also for aurora viewing) - http://www.uphellyaa.org/