Try Tropical Farms aka The Macadamia Nut Farm on Oahu's windward side near Kualoa. The prices are reasonable and a good combination of foods and knick-knacks. Their coffee-coated macadamia nuts are TOO DIE FOR!
If you have time, take in the Ali'i Tour. It's worth the $10, and one of the best tours I've ever done--not too long, entertaining, and educational.