We ended up doing this as one of our MOPs crafts. (See the other crafts we've already done here and here.) It was really cheap and really easy.
Bottle (we recycled water bottles)
Misc. items (to make it really inexpensive, find things you already have laying around the house!)
Hot glue gun
Index card (optional)
Step 1: Gather your supplies.
And there you have cheap entertainment!