Kahana Bay is downright beautiful, and at its mouth is a large sandy beach with usually more room than people. While it's not the most common of spots for tourists, there is a good mix of local residents and travelers stopping by. There is no surf due to the protective bay, but it does have great kayaking and stand-up paddleboarding. Surrounded on all sides by cliffs of lush green rainforest, the area gives you more of a Kauai vibe than an O'ahu one. The Kahana River pours out into this bay, which really doesn't help the visibility in the water, so the snorkeling likely isn't any good. The Pu'umanaman Trail ends nearby, making this a great place to cool off after the grueling hike.
Kahana Valley is one of the wettest spots on the island, receiving upwards of 300 inches of rain per year. As part of an ancient Ahupua'a, the bay was the makai (seaside) portion while the valley was the mauka (mountainside) portion. Together this made one "county" of the island that was self-sufficient thanks to its creative borders. Today the area is protected and has a few trails and cultural and historic features worth checking out if you are looking for more to do than just enjoy the beach.
This beach is not going to win any awards considering the competition, but what it does offer is peace, quiet, and solitude on most days. Located between Kualoa Ranch and the Polynesian Cultural Center, it's another place worth checking out on any east O'ahu road trip. Camping is available; head to this website to book sites.