Crete in December---what to do?

Chania would indeed be a great base for December. Because it has a beautiful old town, Venetian harbour, with its year-round population most tavernas, cafes etc will be open. Some tourist shops are closed now, but many will be open until the end of the year. You can easily spend a couple of days wandering around the town, exploring the tiny alleys off the harbour. As a minimum, you can go to the central market for your wine, oil, honey etc, although if you can find the laiiki (street market) on a day you are here that is wonderful for local produce.
There are a couple of wineries in the Chania area which do tours - Manousakis & Dourakis - although they won't have organised tours out of season, they might open for you if you contact them, you'd need a car/taxi to reach either. & you could visit an olive oil mill, I recommend either TerraCreta (near Vouves, the 2nd oldest olive tree in the world, which has a lovely little museum too) or Biolea - the first is a new, highly technical production, with a tasting room, a very interesting tour, the latter is organic & produced by stone press. Both oils delicious.

If the sun is shining you could easily get a bus to a local beach to stroll, sit & enjoy coffee etc, maybe a swim if you are brave (it's not too cold yet, I was swimming last week). If it rains you have several interesting museums in Chania city.

Many fantastic restaurants in Chania, the food is locally sourced, traditional, healthy, delicious.

thanks so much for your great suggestions. but I think I will go to Crete when I have more time and maybe not in winter. so I will "do" Athens again--always lots to see.