If you are going to be there 2 weeks, I would recommend going to some of the other islands. I've only been to Oahu - but everyone says some of the other islands are even more beautiful.
On Oahu, this is what we did
Wakiki beach
Honalulu shopping
snorkling in the famous bay that starts with an H
Dole pineapple plantation
Cultural Center is a must
Diamond Head
Pearl Harbor
We went to a rain forest right outside of Honalulu - took a guided tour
We rented a car and drove the road completely around the island - cool!
Watched the surfers on the opposite side of the island from Honalulu
That's all I can remember - we were only there 5 days though. I'm sure there is tons more to do and some others will help you out.