Good snorkeling at Hanauma Bay, southeast corner of Oahu. It is a collapsed volcano. There is also Shark’s Cove for snorkeling on the north shore near Waimea falls. The Waimea Falls is worth some time, and I also like walking around Manoa Falls. Manoa is pretty much straight north of Diamondhead.
If you do head to the north shore of Oahu, according to locals, the best shave ice is in Haleiwa Town.
Neat to stop at the Dole plantation from the north shore heading back to Honolulu.
Surprisingly good Mai Tai at Cheeseburger in Paradise across the street from Waikiki beach.
Do Not buy leis off the scammers on the beach.
I’ll try to add more as I remember.
One of the best sunsets you can watch is from Ko Olina on the southwest shore of Oahu. It is mostly resort guests, but you can just go there and swim in the man made lagoons, and there are a bunch of food options right there too.