XMAS/New Years is a good time, but also high season as well. So costs are much higher in normal non pandemic situations.
Weather is about 80-85 degrees fahrenheit in the day.. about 70's in the evenings. Can expect more cloudy skies vs summer and some rain.
So depends on where you are coming from.
I actually like this "cooler" weather as we do more open air hikes during this time. Whereas doing stuff like Ko Ko Crater can be quite draining in the summer time.
You can definitely still lay out on the beach and enjoy rays as well as go swimming in the ocean. You can last longer in my opinion as the sun doesn't melt you. Again, depends where you are coming from.
In normal non pandemic times, things are open. Resorts are open every day.. they don't shut down the pools/food services etc for Christmas.
Pearl Harbor is open except for Christmas. I remember going there Christmas Eve and it was full hours. If you are in Waikiki, restaurants are open throughout the week.. some might be closed for Christmas. As you head outside of Waikiki, restaurants will generally be closed for Christmas.
If you stay on Jan 1st, you can do some interesting shopping. There's a custom in Japan that is somewhat adopted in Hawaii probably due to the number of Japanese Tourists where you can shop for "Lucky Bags" or "Fukubukuro" Can go to Ala Moana Shopping center and some stores will participate. (less stores this year)
https://www.alamoanacenter.com/en/events/fukubukuro.html
These bags are sealed but maybe labeled "for men", "For women", "Large, med, small sizes " etc...
You don't know what's inside them, but they'll list the retail price of the bag and the price you pay is usually half of that (more or less).
You get better value in these Lucky Bags in Japan in my opinion, but Hawaii retailers gives their best shot at it. Used to be better when Apple participated as your bag might be even luckier as some of them had iPads, iMacs, etc as "winning" bags, but they stopped doing that a while back.
Overall, XMAS/New Years is a great time to go, especially if you are escaping the cold. It is high season though. Rates you see now are probably 2X lower than normal due to the pandemic.