They have tons of offerings during the holiday season. We used to go during HHN but decided to go early December a couple years ago and it is our must trip of the year now. Between the decorations, low crowd levels (as long as you go after thanksgiving and before the week before Christmas), Mannheim steamroller concerts, Grinchmas in Suess Landing, there is just so much to see and do.
Yes the pools are open and Volcano Bay is open as long as it is warm enough. December can be fickle with the weather. It’s a very dry month, but it can be 85-90 or it can be 50-60 for the high, and the swing can happen overnight. We are from CT, so 58 for the high was fine for us; but that can be chilly for people used to warm weather.
I can’t recommend it enough, it’s the best time to visit in my opinion.