For me, this is a very cheap and yet very effective Christmas Tree decoration, and it is really easy to make!
Simply buy some lemons, limes, oranges, and grapefruits. Slice them thinly, and lay them on a baking tray with some greaseproof paper on it. Put them into an oven at a very low temparature (around 60-100 degrees celsius), and leave them to cook very slowly for about 12-15 hours. I normally put these on, and leave them overnight. You should also try to turn them over every now and again.
When they are ready, with a darning needle, pierce a hole with the needle, and thread some string, or gold cord. Tie them in a knot at the top, so that you can use them to hang them onto your trees.