Note: This is inexpensive, too. The box shown was $2.99 at Target and is found in the laundry section.
Bend white pipe cleaners into shapes. We used dental floss to hang them (but you can use any type of string or thread).
In a large bowl, add three tablespoons of Borax for every one cup of boiling water. Stir until dissolved. You will need enough water to completely submerge your ornament, so make sure you keep your 3:1 Borax to water ratio as you add more. Hang the ornaments from a pencil or ruler and submerge them in the solution. It is best if they don't touch the sides/bottom of the bowl. It doesn't take long for the crystals to start forming, but we generally leave them overnight.
Try other shapes, too...
This is fun!