Watch the Humpback Whales
If you happen to be visiting between December and April, consider watching for humpback whales. Every year whales migrate to Hawaii's warm waters to give birth to their young and also to breed. These massive creatures can easily be viewed from shore, where you can see their blows as they come up for air, and watch them breach and perform other behaviors. One of the best places on Oahu to watch whales from shore is at Lanai Lookout, which is an overlook with a perfect vantage point for looking out over the ocean.
Honolulu, HI 96825
Snorkel at Shark's Cove
Hanauma Bay is the most well known area for snorkeling on Oahu, but it can be crowded and you would have to pay for parking as well as an entrance fee to the park. If you are visiting Oahu in the summer, consider traveling up to the North Shore to Shark's Cove. It is one of the best snorkeling and shore diving sites on Oahu and has only a fraction of the crowds found in Hanauma Bay. The entrance to Shark's Cove is very rocky, however once you are in the water keep an eye out for tang, wrasse, trigger fish, surgeon fish, eels, turtles and numerous invertebrates.
59-611A Ke Iki Road
Haleiwa, HI 96712
Hike to a Waterfall That You Can Swim Under
If your idyllic dreams of Hawaii involves swimming under a waterfall, then consider hiking to Maunawili Falls. The trail to the falls is located in Kailua on Oahu's windward side. Simply park along residential Lola Road and look for the gate for the trail. The trail will take you through lush tropical greenery, past coffee plants, heleconia, wild ginger and banana palms. The trail is roughly one and a half miles each way with an ascent of 500 feet, however it does require several stream crossings. Eventually you'll get to a waterfall which falls 25 feet down into a small, freshwater pool. Make sure to wear some good hiking shoes as the trail can be difficult to traverse at times. In addition, pack plenty of mosquitos repellant. If you do decide to swim in the water, be sure that you don't have any open cuts as leptospirosis is prevalent.
Kailua, HI 96734