The clarity of the water is fabulous and its very warm. Take advantge especially if you live in a place where the opportunity to go swimming rarely presents itself.
There are about a bazzilion snorkling tours around that usually include lunch. Pick up one of the tourist publications for phone numbers, prices and discount coupons.
Out of the way snorkel spot known mostly to locals, it is on the leeward (west) side of the island. It is known as Shark's cove for a reason - the hot water from the power plant enters the ocean here and attracts many fish which feed on the coral. Occasionaly a shark has been sighted.
Snorkeling at Hanauema Bay, Oahu, Hawaii. This is the most beautiful place to snorkel, but go early because it attracts lots of people.
Polynesia Culture Center, it is a village with reps from all different islands of polynesia. There are shows at different villages, they teach you how to make fire, and there's a big Luau at the end of the day to satisfy your appetite. The best Luau in Hawaii.
Hiking in the rainforest is also neat, you can swim under a waterfall and explore in the forest.
Day or night snorkeling....thats right some fishies glow in the dark....why not go all day, see the sunset then snorkel some more? Locals and military get a discount with proof of residence....i.e. a state license, otherwise I think it's $5 dollar entry for all day, then you can bring snorkel equipment or rent it for a fee on the beach. Parking is 1 or 2 dollars, sometimes free, not sure on that...but it is all worth it. They need the money to maintain the quality of life for the fishies and protect them by limiting human damage to the reefs.