Although many fruits and vegetables can be molded as they grow it is most often done with cucurbits-melons, squashes, cuccumbers, and gourds---because of the drama of their size, and the unexpectedness of the shapes.
Cubical watermelons became a media darling a few years back when the Japanese started doing it because a cube fits in the fridge better than the natural shape. That didn't capture the hearts of the media folk so much as the pricetag.
You can buy grotesque molds and create halloween decorations with them. Cukes, gourds, mini-pumpkins all lend themselves to this. And kids really love veggies that have faces in them.
And a really fun project is to construct a set of boxes that increase in size and grow gourds in them. Then convert them into a set of graduated canisters. In the past I've gotten as much as $150 for sets like that.
Molds do not have to be cubical, btw. Any shape you like is possible, so long as you make the mold in multi-parts. Otherwise you won't be able to remove the more complex ones.
They have taken the oath of the brother in blood, in leavened bread and salt. Rudyard Kipling