Today was spent at Waimea Valley, swimming under a waterfall; at Waimea Bay, swimming in the sea with a turtle; and at Sunset Beach, watching the surfers ride the waves. It was a beautiful, relaxing day. Waimea Valley is famous for its waterfall, which (after paying $13 to get in) anyone can swim under!
I would recommend it to anyone; however, a few hours after leaving, I found one of the free maps I had picked up (of North Shore O’ahu) actually did contain a coupon for buy-one-get-one-half-off admission. So if you go, make sure you get that coupon!