Hanauma Bay Nature Park is one of the more popular recreational swimming, snorkeling and picnicing areas on O`ahu. The bay itself is legally designated as a State Underwater Park and Conservation District.
The bay is the result of the ocean's breaching and incursion into two volcano craters. There are hiking trails with very pretty vistas and the park areas have good picnicing sites.
Avoid walking on the rocky ledges where waves are breaking, they can be very dangerous. The rocks are actually lava. Lava is very porous and crumbles easily.
Hanauma Bay Nature Park closes down all day on Tuesdays; all other days it's open from 6am to 6pm. There is a US$3 entrance fee if you are a non-resident; free entrance if you can show valid local identification. There is a US$1 per car parking fee. You can call +1 (808) 396-4229 for other information about the park twenty-four hours per day, including information on emergency closures.
The Hanauma Bay Education Program teaches all about the bay's natural state. If you are going to be snorkeling or diving you are required to sit through the educational movie, which is actually quite entertaining.
Because you have to walk down to the bay from where you leave your car, it is possible to take a bus down, we walked down but took the bus up, well worth the $1 one-way fee, especially if you're carrying a lot of equipment.
A couple of miles further down Kalaniana`ole Highway, you'll come to a spot where there are invariably a couple of cars pulled over to the side of the highway. There is a great hike up the hill, with phenomenal views, including a light house and I understand if the weather is good you can see some of the other islands.
There is no cost, although I'm not altogether certain that the hike is legal, but it a lot of fun, though a certain level of fitness is required. Since the wind is very strong, I don't suggest wearing skirts.