The east side of Kauai isn't called the Coconut Coast for nothing. The term does have its historical roots: the coco palm was one of the 23 plants the Polynesian voyagers took with them in the "canoe garden," when they faced 2,500 miles of open water to sail here from Tahiti. The ali'i (royalty) planted groves on this coast, many of which remain from the last days of the monarchy.
But fresh coconuts are all about today in Kapa'a, as well as other towns and rural areas in east and north Kauai. For a couple bucks, a machette-wielding gardener will whack the top of the coconut so you can drink the water with a straw. The water is highly nutritious, even used for IV's when medics ran out of plasma in the South Pacific during WWII. The meat of the coconut is soft like a melon at this point, and can be eaten with a spoon.
The brown-nut coconut needs to dry for awhile before getting its nutty texture. Those are available at stands too. These babies are hard to crack. Needless to say this island staple is only one of many, many fruits and veggies available at roadside fruit stands. If you want to eat local, Hawaii is the place to be.