I believe the criteria is:
Items that float. The amount does not matter.
Lead, potatoes, ripe blueberries, and metal all sink in water. I couldn't find a consensus on whether brains float in water hence the "probably not allowed". Cooked Ravioli, apples, bread, zucchini, grass, Bananas, and pizza should all float. Regarding the second hint, you are going to a deserted island, so you probably want things that can easily float.