Remember: tinsel on trees can hurt cats. They tend to try to swallow it and it can get wrapped up in their intestines and cause all sorts of grief and damage. One of the ways I avoided tinsel for many years (before finding other ways to just do away with it altogether) was to buy those long strands of larger metallic beads and cut them into 18" lengths. These are wonderful for draping over tree limbs and they give the sparkle of tinsel but cats tend to leave them alone. Even if they bat at them, they can't easily swallow the strings and so you avoid the problems of tinsel and still get a similar effect. 

Be cautious when putting out chocolate treats--chocolate is poisonous to cats and it doesn't take much to hurt them. They tend to like it, too, I've noticed, or my own cats do, so be careful. 

If you have a real tree instead of an artificial one, make sure they don't drink the water in which the tree is sitting. I don't know if this is dangerous or not, but it probably isn't a good idea.

If your cats tend to play with the ornaments, put the unbreakable ones on the bottom. I like to hang bells on the bottom limbs of our tree (which is artificial) and when the cats sneak beneath it, they ring the bells and it sort of adds to holiday spirit.

During holiday dinners, watch out for bones. Don't leave your turkey or chicken bones around, and watch out for lamb chop bones too, which can be sharp. Don't let kitty have many bites of people food--the best idea is, if you want to give them a special treat, by kitty treats or really delicious canned food and give it to them for their holiday meal.

Ribbons can be dangerous--cats can swallow them and again, like tinsel, it can get wrapped up in their intestines and bring them to grief. (Same with any thread or dental floss). If this happens and you see the end of a thread or ribbon poking out of their butt (to be blunt), do NOT pull...take them to the vet because it could be wrapped around something inside and you could hurt them by pulling.

Your best bet is to take all ribbons off the packages before you put them under the tree, or use very thick ribbon that lays flat against the presents.

Some links with further information:

Holiday Hazards
Humane Gifts For The Holidays
Holiday Handouts (video clip)
 

Here are a few Kitty Yuletide Carols for you and your cats to sing (yeah, I know they'll just warble along, that's okay as long as they're having fun).

O Kitty Treats

O Kitty Treats, O Kitty Treats,
How sweet you are to eaaattt...
You crumble nice inside my mouth,
You were once a bird, once a trout....
O Kitty Treats, O Kitty Treats,
What a pleasure you are to eat....
 

Silent Night

Silent night, bat the lights,
bat the balls that shine so bright.
Curl beneath the indoor tree
Wonder what presents are waiting for me
Tear the paper, eat the bows
Then groom my butt and lick my toes!
 

Merry Kitten Folk

Bast, rest ye merry kitten folk
Let nothing you dismay
Tis time to romp and race about
And cause such disarray!
Tis time to eat the ribbons 
on the packages so nice,
Good mousings to all you, good mousings to all
Good mousings to all you good cats...

Never fear that winter's here
The food bowl is always full,
And when you want to play around
Just grab a cord and pull!
The lights may blink, the tree may fall
Just run away and hide!
Good mousings to all you, good mousings to all
Good mousings to all you good cats!