I've dove off Kauai, Oahu, and the big island and I conquer, the big island is the youngest island and by far the best diving, and the night dive with the manta rays is one of my top dives ever. Also when I was there, we went to see the live volcano (by land) and there was a dive boat doing a night dive watching the lava flowing into the ocean from underwater. That looked amazing. I inquired but only had a couple days left and never found out who was doing it. I want to go back just for that opportunity if it's something that's being done regularly. Something worth checking out for sure if the volcano's flowing and you've got the time.
If you do do find out about the night dive watching the lava flow into the ocean, please let me know. I'll totally fly back just for that.

cheers and enjoy.
-duncan

Thank you so much for the great recommendations. My trip starts tomorrow in Kauai :-)