Stacks of crates. Both empty and full. I'd imagine an island prison would require a steady stream of cleaning supplies, food, office supplies, clothing, lightbulbs, and any raw materials for prisoner work programs such as making license plates or bookbinding. Assuming that either RED or BLU (or both) have taken up residence and are using the island for some nefarious purpose, such as a mind control laser aimed at the mainland or an operations center, you'd probably have need of other things such as electronics gear, construction equipment, and so forth.
Could probably get away with adding a few giant spools of cable, and maybe a front end loader or forklift as well. Add some pallets and you're all set.