One can't miss activity/restaurant in Oahu
 

If you had to choose one activity/restaurant in Oahu that can't be missed, what would it be and why? Thanks.

Snorkeling in Hanauma Bay - Very relaxing, can be done at your own pace, beautiful fish, sea turtles etc.

I would say sunset Mai Tai's at the Halekulani Hotel in Waikiki. They have a wonderful lounge called the "House Without a Key" that is arranged around a 100 year old tree and has perfect views towards Diamond Head. The drinks are superb...not the syrupy sweet garbage you will find at the very tired Royal Hawaiian. They also have an excellent and very classy hula dancer named Kanoe Miller...she was a former Miss Hawaii and performs nightly. My favorite spot on Oahu!!

Sunday brunch on the lanai at the Moana-Surfrider. Get the strolling musicians to play the song "Waikiki" for you and reserve by phone..weeks ahead!

Duke's breakfast buffet.
Sunset dinner at Roy's.
Genki Sushi.

Turtle Beach on the North Shore, where you can walk into the water and be surrounded by and swim with big sea turtles. They even come up onto the beach to sun themselves, just like people!

Wow, what a hard question! There is so much to see and do on Oahu.

I definitely agree that you should not miss snorkeling at Hanauma Bay, watching a sunset at the Halekulani's House without a Key, or breakfast at Duke's. All three of those things were fun and highlights of my trip.

BUT, the only thing I would say absolutely can NOT be missed is a visit to the USS Arizona Memorial. We had an actual Pearl Harbor veteran do our intro to the film they show you beforehand, and he was really great. And the memorial itself is something I'll never forget, especially the wall with all the names of the men who died there. Truly moving and something I think about often.

A hike to the top of Diamond Head.