Oahu is a manageable island — almost everything is within an hour's drive. Hanauma Bay on the eastern coast is the top snorkelling destination, requiring early arrival and advance reservations. The North Shore (Haleiwa, Pipeline, Waimea Bay) rewards a full day driving the coast and stopping along the way. Kailua on the windward side is the island's finest beach, with kayak rentals to the offshore Mokulua islands.
Further afield, the Ko Olina resort area on the west coast has lagoon beaches that are calmer than Waikiki and uncrowded. The Byodo-In Temple in Kaneohe valley is worth combining with a Kailua day trip. For multi-island trips, Maui (45-minute flight) and the Big Island (45 minutes) are the most rewarding day-trip or overnight destinations — each has things Oahu does not, including active volcanoes and the Haleakala crater.