Planning a trip to Oahu and Kauai and would like to use that for my first dive. I'm already certified so no time needed for that. Any thoughts on which location might offer the best opportunity?

For a very diversified first dive - and safe and easy - consider Hanamalu Bay - just south of Waikiki on Oahu.

There are many other good dive spots on both islands but that IMO would be a great intro dive.

ddubya,
If you are a new driver under 25, make sure you check with the car rental agency as some have restrictons. As a new driver, both islands would offer challanges. Overall would suggest Kauai

For some reason, I read driving, Im sorry

great instructor/dive master off of Tunnels beach north shore Kauai...i send everyone to him if on the north shore. His instruction/ dives don't have time constraints and allow for ease of learning and experiencing. Poipu side is great as well...outfitters are top notch and know the areas/hazards/vibrant sea life
Hanauma bay south side Oahu is for sure safe yet overrun with bus loads all day everyday. If summer time, head to Ke Iki beach right past Waimea bay Oahu and the reefs are awesome! two companies take small groups there...

My wife was Scuba certified in Hanalei Bay during our Honeymoon back in '86. The company was called Bubbles Below - and think they are still in business - but don't know if it's the same owners/dive masters now.

Wanted to get out and dive a small - crescent moon shaped island off Niihau - and they told me after our Catmarin cruise/dive up the Napili Coast (we left out of Port Allen on the south side) - sure - no problem - come back tomorrow - but when I came back - some wedding group had chartered the whole dang boat.

Dems da breaks bro.

We used Fathom Five Divers in Kauai and went to Sheraton Caverns which is ok for beginners and of course Tunnels Beach but I think that might be a little more difficult for a beginner. In Oahu we did the Corsair dive but that is for advanced divers, used Island Divers in Hawaii Kai.

Thank you to everyone that has already posted and who may post future messages to this thread.