I know from experience that saltdough decoration have lots of different stages that take days, so that might be a good idea for you! Lots of mess too, which will please the DC 
Day one - make the saltdough, roll it out and cut it into shapes with cutters or knife (stars and christmas trees are easy, but you could do bells and snowmen and reindeer if you're creative). If you're going to hand them on a tree, rememeber to make a hole near the top to thread some ribbon/string through. Leave to dry out - they take a couple of days
Day two - paint them. I used ELC silver and gold paint (£1.50 for a big tube) last year, but you could use any paint
Day three - glaze them (I never bother with this)
Day four - decorate them with glitter and whatnot and put the ribbon/string through and them hang them up.
I have also used the reverse side of a roll of wallpaper and sellotaped this to the kitchen floor so it is maybe 7 feet long (or whatever the height of an internal door is). Draw a Christmas tree shape in pencil. put some green paint in a tray. Let the children step in the paint and paint in the tree shape with their feet! When that is dry, they can decorate the tree with pop poms, tinsel, ribbons, whatever tat you have. And then hang it onto a door. Instant decoration 
Borrow Christmas books from your local library. Go now, so that they aren't all taken. You can always renew them later on.