The one date to check first
A December weekend up here works beautifully as long as you know the one date that trips people up. The Catskill Fly Fishing Center and Museum closes December 9 and does not reopen until January 9. Come the first weekend of December and it is open. Come the third and it is not. The grounds along the Willowemoc stay free and walkable either way, and a frozen edged river in December is worth the trip on its own.
Bethel Woods keeps the museum open through December 31, so it is on the table for the whole month right up to New Year's Eve.
December is also when the Callicoon winter market starts, which is the reverse of the problem October and November visitors have. Finally, a market to go to. Plan a Sunday morning around it.
Breweries: all three are open and all three are good, and we say that having been to each. We are not going to tell you which to pick in December because the honest answer depends on the day. If it is bright and you want to be outside, Upward has the outdoor space and it is the strongest of the three for that. If it is grey and blowing, pick whichever is closest to where you already are and settle in.
A weekend, laid out
Here is how we would build the weekend. Friday evening, arrive, eat, go to bed early because the dark comes fast in December and there is no shame in it. Saturday, the river in the morning while it is cold and clear, a brewery in the afternoon, Bethel Woods if the weather turns. Sunday, Callicoon market, then home.
That is a genuinely good weekend, and you will spend most of it not queuing for anything. Bring layers, check the Fly Fishing Center's date before you commit to it, and come have fun.