The easiest thing, of course, is to just glue a Christmas card onto a box. This works great if you have a really special card, perhaps one that is hand made or really unique and a box that is the right shape.
You can get a little fancier by putting pictures on the top and the bottom. This works well for small jewelry boxes.
First find a card that you think would make a nice box. I look for ones that have a nice image mostly in the center or ones that have an overall design. Then cut the card in half. It is a good idea to trim the bottom piece just a little bit smaller, like an eight of an inch on one long and one short side. This way the bottom ends up being a little smaller so the top and bottom fit together more easily.
Next, where the lines intersect on the short sides cut a slit up to the line. This makes the tabs needed to glue the side together.