MadameJ some more ideas for calendar. i will also use some of Like silver's.
my kids are older than yours now. Like Likesilver says get out your calendar/diary and plan around stuff you have on already and your busy and quiet days. obviously takes a bit of planning but once it's done we have a lot of fun.
we have some little paper triangle boxes with numbers 1-24 which we tie onto a bare branch in a pot. i'd like to say I made the boxes but I got them off ebay. lots of great ideas online but you can always peg little envelopes on a string or pin to a board if time/energy in short supply.
We have a mixture of tasks and treats
Tasks
Buy tree
Decorate tree
Make/write cards
Get house ready for Santa (clean and tidy rooms etc)
Go shopping with Dad to buy present for mum
Wrap family presents and put under tree
Go for winter walk and collect greenery for table/mantelpiece
Lay table on Christmas eve etc
The treats in the calendar can be
elaborate - panto, iceskating, see Santa, trip to cinema
Easy- some jelly beans or buttons or shiny pound coin pound coin to go to local shop to choose treat
really simple - hot chocolate, watch Christmas film, make paper snowflakes, play board game, Christmas story
When the kids were smaller I had a few Christmas story books that came out with the decorations and went under the tree and got put away again ready for next year.
Of course any carol services/concerts/school parties can have a slot too giving you a day off. Don't stick down envelopes in case you have to do last minute swap due to weather/you can't be bothered. I have a typed list so cut up the sheet of paper and put in boxes and then I have a master list so I don't forget what's in the boxes!
Phew! Sorry very long!