Pearl Harbor. Book the USS Arizona boat ticket ahead. Go early, and give it more time than you think.
Lanikai and the windward coast. The postcard beaches, the Mokulua islands offshore, and a completely different feel from the south shore.
The North Shore in one day. Haleiwa, the shrimp trucks, Waimea Bay and the turtles at Laniakea. It is a full day, not an afternoon.
Diamond Head at sunrise. Reserve it 30 days out. The crater rim at first light is worth the alarm.
Hanauma Bay. The best easy snorkeling on the island, if you can get a reservation. Closed Mondays and Tuesdays.
Eating your way through Honolulu. Poke, malasadas, plate lunch and some of the best Japanese food outside Japan. This is the island for it.