make them clean the whole house :P just kidding ... open for them TV make them draw or paint , or make them sit on the computer and show them new games or virtual games alot of stuff to do or you can play kings if you know it .... good luck
A lot of good suggestions already written. You can also ask their parents what kind of things they enjoy doing. It will make you look good as well as get some guidelines like computer restrictions, etc.
Sit down with them and craft Christmas decorations. That should get them excited. I have a step by step guide to easy Christmas heartbaskets for the Christmas Tree here http://www.squidoo.com/paper-heart-craft if you need inspiration. :)