I buy plain colored Christmas bulbs. I usually get silver, gold, blue, purple, green, etc. Then I buy the glitter glue tubes, it's WAY easier than buying glue and glitter separate (and messier), rhinestones, ribbon, and other Christmasy decorations.
I give the kids the bulbs, glitter glue and decorations and let them go to town. They come up with the prettiest decorations you've ever seen. WAY prettier than anything you can buy!!
If they make a mess, it's just glue and can be wiped up. No big deal!
I usually write a year (or have them do it) somewhere on the bulb so when we pull them out year after year, we'll know how old they were when they created it.
It is great to pull them out of the box to hang on the tree and you see ornaments from when they were 5 or 6 and how much better the artwork gets as they get older.