Don't over load it. Have 1 activity per day - there will still be homework needing doing and dinners to prep and laundry to tackle throughout December. But yes to having plans in advance.
Make them flexible if you can. Some things need to be pre-booked, but things like crafting could be set aside one really fine day for a walk outdoors instead. Or if you hear of an event locally at the last minute, you can drop and run.
I keep a shoebox of strips of coloured paper for DD to make paper chains in December, they decorate the hall coming up to Christmas itself, but is something that she can do for 1 minute or 45 minutes at a time if she needs something to do.
There are lots of days we can do things together though. If DCs are young-ish, I wouldn't plan on being ambitious about baking - make cookies one afternoon, decorate them another -that sort of thing. They may not have the staying power to do it all on 1 day.
I also usually have a bundle of printable colouring sheets and seasonal puzzles from the web for when I can't get involved but DD needs something to do - there are lots of maths, reading and practicing writing type sheets that are so seasonal and fun that DD never realized she was doing "homework" type stuff. And some are pure fun.
There's a website called "Santa Update" which has lots of news stories from the Elves at the North Pole, and tracks Santa on Christmas Eve (different timings to NORAD's tracking system - so just beware of that - but you could explain away by saying 1 has better technology than the other!). It could be good to have as something you take a look at every few days.
A "carpet picnic" could be fun on a rainy or cold afternoon with no other plans. Lay a rug on the sitting room floor, get a drink (and maybe a small treat?) and all sit on the floor on cushions to watch a movie together.
And there are lots of Christmas stories to get from libraries even if you don't have a stash of books yourself. Have an afternoon choosing them, and then times when you all curl up together reading your books by the fire and allow gentle chat to happen.
And get the DCs to help out a bit too - its a great time of year to instill good habits (as you never know what Santa or his Elves may be watching, and checking the Naughty and Nice lists!!), but you can make chores fun and do them together - so all fold clothes together in the sitting room (and you show them how as you work with them) before you can get the rug out for the carpet picnic; or they use the small brush and pan while you use the big one to sweep the kitchen; or they put the old towels in the hamper and hang fresh towels while you finish doing the bathroom.
And lots of music - varied between the Christmas classics and cheesy tunes, some generally favourite music, and other seasonal music that is not all pop/rock - carols and choral music is really lovely and can set a different mood when you are working away companionably on a project, but then change to the pop ones for a dance around the kitchen when the quieter stuff is done!
Enjoy!