What can be done with the little plastic capsules from Kinder Surprise eggs?
The capsules used to be two-part, but have recently switched to having a living hinge attaching the lid section. They're a little under two inches long and about an inch in diameter (the pic in the above link shows the older, two-part design - but they're the same size either way)
Surely there's something useful that could be done with these little plastic containers - they close pretty tightly - so I guess they could be waterproofed pretty easily (LED 'floaties' maybe? - although releasing them into the ocean would be littering)