"Duty-free" is a magical term here. Major credit cards are welcome virtually everywhere, U.S. dollars are accepted almost as readily as local currency, and traveler's checks can be cashed with proof of identity.
Aruba's souvenir and crafts stores are full of Dutch porcelains and figurines, as befits the island's heritage. Dutch cheese is a good buy (you're allowed to bring up to 10 pounds of hard cheese through U.S. customs), as are hand-embroidered linens and any products made from the native aloe vera plant -- sunburn cream, face masks, skin refreshers.
Local arts and crafts run toward wood carvings and earthenware emblazoned with aruba: one happy island and the like. Since there's no sales tax, the price you see on the tag is what you pay. (Note that although large stores in town and at hotels are duty-free, in tiny shops and studios you may have to pay the value-added tax of 6.5%.) Don't try to bargain. Arubans consider it rude to haggle, despite what you may hear to the contrary.