Ben Said,
First off you need to take a submarine excursion. You will see lots of tropical fish, coral, reefs, sunken ships and planes. It is really a wonderful side trip and the whole family will really enjoy the view and the submarine ride.
Sometime while on Waikiki you will want to stop by the International Market Place for souvenirs. It is a whole bunch of outdoor shops selling items to remind you of Hawaii.
You may want to wander through Queen Kapi'olani's garden, it is a beautiful tropical garden and worth seeing.
Now if you want to learn to surf then Waikiki is the place to try it. I recommend the Waikiki Beach Boys, they will have your whole family surfing in no time (safely).
If kayaking is more your speed then Kohanamoku Beach would be the place to go. If you are lucky enough to be their on a Friday night you will be treated to a wonderful firework display.
If snorkeling is what you want to do, my recommendation is to do it off the white sandy beach of Ft. DeRussy Beach Park.
Another fun thing to do is take a catamaran ride and sail to Diamond Head.
One can't miss is if you are in Waikiki on a Monday night then you must go to the Royal Hawaiian Luau. Wonderful food and entertainment. Truly a highlight of any vacation to Oahu.
If you are going to watch a sunset then Kuhio Beach is the place to be. You will also see a torch lighting and a hula show while you are there at sunset.
One other place to mention is the Waikiki Aquarium, it is a really nice aquarium and you will see a lot there.
