Young humanoid had done it. He had constructed a four-item vending machine with coin differentiation (quarter, nickel, penny, dime), coin return, coin sorting and change. All using one NXT, and one PF battery box. Young humanoid covered over the front of the machine with clear plastic sheet that allowed users to view the products, which fell from four independently operated treads. The user could decide which product he wanted from the on-screen interface, and then enter in any coin combination he desired. Then, the applicable change would be calculated and dispensed. The largest problem with the machine was the frequent jamming of the coin slot. However, young humanoid considers this his largest (well, that's no question) and one of his best projects yet.
