A few things I remember from my 2/c MIDN cruise...
As others have said, Lanikai and Kailua are great because they are beautiful, less-crowded beaches. Also, you can kayak to the "The Mokes," or the twin islands off the beach at Kailua. Huge sea turtles out there.
I only surfed once, and it was at White Plains. If you don't like locals giving you the stink-eye and calling you Haole, I wouldn't go there.
If you like a long hike and jumping off a ~40ft waterfall, then you should do the hike to Maunawili Falls. Another good hike was Diamonhead, and it had a great view at the top.
Waimea Bay is awesome. Nice beach with a large rock for jumping and good swimming. Also, if you go further up the road, you will find a few good shrimp shacks. I highly recommend Giovanni's.
There are also the usual tourist spots such as the Dole Plantation and its pineapple ice cream, the Shave Ice (not shaved), and Waikiki.
Some other things:
-Do not hang out with the girls wearing clear heels in Waikiki lol
-If you must try the Stairway to Heaven, start early in the morning before the security guard gets there