A few more that occurred to me today (it was slow at work)
Fill them with small rubber snakes, frogs, etc. Make battery operated mobiles and sell them on Ebay, what herp lovin' Mama wouldn't want one of those for her new bundle of joy!
Fill them with aspen or sani chips, mark the lids with a ? and set them on your table at the next expo. Watch them disappear.
Construct the world's tallest free standing deli cup tower. Call "The book of records" people, not the brewers.