For the tourist:
For the kids:
KCC Farmers Market (Saturdays only) One of the best collections of local food, vegetables, plants, and coffee. Also, you can get a good breakfast here.
Hanauma Bay (note:this link is for the tours, not the park) This beach is declared a marine life sanctuary and underwater park. Lay out on the beach or snorkel with the fish. Feeding the animals is prohibited. Closed Tuesdays. Located near the south east tip of O'ahu.
Halona Blow Hole One of O'ahu's natural wonders, but bring the sunscreen and water. You might wait awhile to see it spout. On the east side of the island past Hanauma Bay.
Makapu'u Lighthouse Trail Hike to one of O'ahu's historic sites on the east end of the island past Hanauma Bay and Sandy Beach.
Byodo-In Temple Memorial Park Scale copy of a buddhist temple in Uji, Japan built without nails. Visit the park and the impressive non-denominational temple along the way to the Pali Lookout. Some episodes of Magnum PI and Hawaii 5-O shot here.
Pali Lookout Small lookout with a great view of Kailua Bay and site of a historic battle that united the Hawaiian Islands.
Punchbowl National Cemetery One of the more scenic national cemeteries with several famous "residents" like the war correspondent Ernie Pyle.
Tantalus Lookout Lookout on Tantalus Mountain in Manoa Valley with a great view of Waikiki, downtown Honolulu, and Pearl Harbor. In Pu'u Ualaka'a State Park, but locals just call it as Tantalus. Located near Punchbowl.
USS Missouri Memorial Decommissioned battleship now a museum.
Pacific Aviation Museum Small museum with a growing collection of aircraft.
Honolulu Academy of Arts Small museum of Pacific art. Know for their collection of Japanese wood paintings.
The Contemporary Museum Smaller museum of contemporary art in Hawaii.
Shangri-La Former home of philanthropist Doris Duke, now a museum for Islamic Arts on the southeast side of Diamond Head. Book tours through the Honolulu Academy of Arts.
Polynesian Cultural Center This screams tourist trap, but it's fun. Sign up for the lu'au and the shows.
Aloha Tower As seen in Hawaii 5-O, see it for the Tower and if you want to get a drink at Gordon Biersch or wings at Hooters. Very touristy here.
For the kids:
Hawaiian Fire Surf School Surf school taught be off-duty fire fighters in small groups. What could be safer? And this is for both the kids and the big kids.
For the big kids:
Kualoa Ranch Horse and ATV tours Take a horse back or ATV tour on the north east side of O'ahu, Jurassic Park, Hawaii 5-O, and Lost shot on location here.
Scuba Diving (note: DO NOT scuba within 24 hrs of getting on a plane! Bends will result) I don't recommend any one particular Dive Shop. There are several good ones I've used over the years, but this is a start point.
Honolulu Marathon Only if you come in December and are willing to run 26.4 miles. Just throwing it out there.
Star of Honolulu Cruises If you want to do a sunset or evening cruise, whale or dolphin watching, or New Year's Cruise (if you're here). If you get seaside bring dramamine, bonine, or those accupressure wristbands.
Atlantis Submarine Tours If you want to do a submarine tour or another Waikiki cruise.
For the shop-a-holics: