If you notice in the images, we used a variety of things designed for puppies (chew bones, squeaky toys, crinkle toys, etc.) as well as some baby toys and a variety of common household items. Anything that did not already have holes for us to use had a few drilled in so we could affix them to the pipes with zip ties.
Remember that these items can be anything you want to expose the pups to, just as long as they are a variety of different materials. By using some metal items along with the plastic, cloth, and foam, not only would they get to see and feel new material, as the puppies played with it, the cube would rattle and make noise. This further exposed them to different sounds in a positive, safe, playful environment.