We just spent two weeks on the Big Island in August-September. It was absolutely fantastic and incredible diving. I did a lot of snorkeling and scouted some shore diving for next time. I have to second the votes for Place of Refuge/Honaunau (Two Step). Just make sure you find the little lava bench for entry/exit and you're set. Other entry/exits can be a drag.
I went out on a number of dives with Jack's Dive Locker and I would never go with anyone else. They have an incredibly friendly and helpful staff. They take you to great places, and are in general a lot of fun. They have a ton of return customers it seems, and the whole thing feels like a happy family.
The only mantas I saw were during the day (one just off Honaunau while snorkeling of all things). They were a no-show on the night dive. But the night dive was still amazing. We saw tons of hunting morays of all types, stonefish, crabs, octopus, and all sorts of other cool stuff. It was well worth it just to see a moray snatch a yellow tang, wrestle with it for a while, and then see the tang escape unharmed.
No one has mentioned Kealakekua Bay, which is a marine sanctuary with a ton of spinner dolphins. I only snorkeled it, but I'm definitely diving it next summer when we return.
FWIW, if you're staying on the Kona side, the volcano drive is FAR. And at least when we were there, you could only see lava at night. If you want to do that, I'd consider getting a room in Volcano village. Then you can explore Hilo, Waipio or whatever the next day.
Big Island Revealed is an *indispensible* book. We had it on us at all times. They turn you on to good food, things to do, places to go. Another book I was glad I had was Snorkel Hawaii by Malinowski. It's pretty snorkel-centric, but can turn you on to some good shore dives too. Their opinions on what is a good spot seem very accurate, and they give great directions to places like the free parking at Honaunau and where to enter/exit.
OK, I'm getting jealous now
Have a great time in Hawai'i and say hi to the Humu's!
Gregg