The diving in Kona is so good that almost anywhere you can get to the shore, and get in and out safely, is a great dive. If the seas are down you're only limited by your enthusiasm and diligence in seeking out access points. You could dive every day for a month and not exhaust the opportunities within an hour's drive north or south of Kailua. Honaunau/Place of Refuge, Old Airport (west end), and 4-Mile are good starting points and the first two are often diveable even when the seas or winds are up a bit.
I like Big Island Divers for rental (and Manta dives!) - $7/day for a tank and around $4 for a refill. They may even have a better weekly on the tank, I can't recall. I always rent two tanks apiece for the time we're there - less hassle scheduling dives that way.
Bonus with the Manta dive: the Undulated Moray snaking it's way in and around you and your gear - will you even see it?
17 hours and counting...