Useful things would be portable chargers, USB charging block, metallic hooks, a pop-up clothes hamper, or travel toiletries bag.
Fun things would be magnets for their stateroom door, clothes, hats, gift card for the cruise line, lanyards for their cruise cards, or themed gifts for where they're going.
I agree that food is not necessary, but a bottle of wine or some beer would be nice if they don't have a drink package. Bottled water is also useful if they're doing excursions and want to bring it.
I like the CruiseTipsTV
Amazon shop for cruise-specific shopping.