One of the most obvious and easy ways to use your old Christmas cards is to make gift tags from them. There are a couple of ways to do this.
Sometimes you can use the entire front of a card as a tag, if it has enough blank space to write the to/from information. These are especially nice for family members -- for example, you might have a card front that says "Merry Christmas to a Dear Granddaughter." This is a natural to paste to the top of your granddaughter's Christmas present. These can add a real decorative element to a package wrapped in a solid color like red, green, blue, silver, etc.
The other way is to cut shapes from your old cards to make tags. You can make simple shapes like rectangles, squares or circles. Current, Inc. sells a set of tag templates which work very nicely and has quite an assortment of shapes. A book of children's Christmas stencils would also work well. Once you have cut your shapes you can punch a hole in each one and add some red or green string to make a hanging tag.
This simple project is a fun way to keep your kids busy during Christmas break -- and I personally find that doing a project like this, even all by myself, is very relaxing and a great stress reliever.
Have fun with this idea!