I had a look for a pirate/mermaid/superhero themed pull string piniata at our local store, but all the ones with the right theme were the beat the living hell out of it variety, so I made one the night before the party instead.
There was no time for papier mache nonsense, so the body of the treasure chest was made from turning a big big multibag cereal box inside out. Then I attached another bit of cardboard to form the curved lid. The pull string mechanism was cobbled together from a polystyrene plate and the plastic lid of a coffee cup. You can kind of see from the pictures how it's put together.
I cut a hole in the plate, big enough that the cup lid would sit in it snugly, then tied the "winning string" onto the spoon hole of the lid, then threaded it around the plate and back through the spoon hole. All the other strings were taped loosly to the outside of the plate and fed through the spoon hole too, so that only the winning string would pull the plate out and leave a big hole in the bottom of the piniata for the presents to fall out of.
Paul and Jessen had the lovely idea on the day of tying the piniata to the top of the slide, so each kid could choose a string, hold onto it and slide down the slide to pull it, then the winning string made all the sweets and toys cascade down the slide too. Lots of fun!