While it is not cheap anymore, I still think the catamaran rides are worth it, especially if you have a cocktail or three on board. I like the red and yellow one. Spectacular views of Oahu.

Although you don't have a car, if you do find yourself nearby for whatever reason, Malasadas at Leonard's is a must. https://www.leonardshawaii.com/home/
Plate lunches, either from a food truck or a hole in the wall is great.

If you have time, the other side of the island is worth it. I think there is a bus that can get you to Kailua. Great beaches.
I would skip Ala Moana mall which was on the list of the perfect 2 day.
If you find yourself after 5pm but before sunset near the Ala Wai canal, they usually have paddle clubs (or at least did, it has been several years since I was there) that is interesting to watch.
Drinks at Duke's at the Outrigger is a must, as it drinks at the Royal Hawaiian. Breakfast at the Westin Moana Surf-rider Veranda at the Beachhouse is a relaxing way to spend a morning. Kona coffee there is great.