Kapoho is fabulous for a snorkel but it's way down by the southeast tip of the island, below Hilo. That vicinity regularly has rough seas so I'd pick low tide for that spot. The tide pools extend out far enough that there are plenty back away from the surf even when it's rough.
Big fish are everywhere in the Kona vicinity, especially where you can find deep water right at the shoreline, off the lava bench. Try Honaunau/Two-Step (20 miles south of Kailua) for a great snorkel or scuba, lots and lots of coral. It's one of the few places that's doable when other places are a little rough, and it's among the best to boot. Swim out to the left 100yds or more and look for big fish up along the shoreline walls. I recall seeing lots of chub along the dropoff, including the cream-colored ones, at Kealakekua Bay/Cook's Monument. You have to take a boat or kayak to get there though. It's also a very protected spot, and great for a snorkel or dive
There are a couple of other good spots for bigger fish closer to town but the entries and exits require a little more care. Try Old Airport if you get a chance, down at the west end of the runway, enter at the cove just beyond. You can go out/left, or around to the right. I'd recommend left and out for the first dive. Lots of cool stuff among the many lava canyons and sandy patches. On one of the sandy patches, we had a huge mu/Big Eye Emperor - one of my top favorite fishes, and usually very wary - hang right with us as we were poking around the sand. It was fascinated with my dive buddy and stayed just beyond arms length. Very unusual for those fish.
If you do a boat dive with one of the Kailua outfits like Big Island Divers, and conditions are decent, odds are high that they'll take you to Konahaiki, just north of the boat harbor. Lots of big fish there and great exploring. Make sure they take you to a couple of the many caverns there up along the wall, not just the flats. I regularly see dive groups poking around the rather unremarkable flats without going up into the canyons and caverns, yet they're typically quite tame.
I see you asked about dolphins and boat rides. If you want to have a nice snorkel exploration trip, try one of the many zodiac-style small boat outfits. You might try asking if they're going to do Red Hill and down to Kona Paradise - great 'cliff' scenery and a couple of sea caves. Sometimes they end up at Honaunau/Place of Refuge, which can easily be done from shoreline, or at Kealekekua Bay. The dive charters don't typically go that far south these days, unless you pony up about $200 for a long range trip. I don't recommend a boat for dolphins, those buggers are too smart for that. There are a couple of places where the odds are high of a great dolphin time from a shore entry, but you might have to try two or three days. If that's what you want, ask around for the good spots.
Since I can't be there, I'll just go on and on. Definitely scuba for the manta dive. While I'm sure the snorkeler's get almost as good of a manta show, the divers get to partake of everything else that's going on, at least the ones that pay attention. Look out and around, it's not just mantas at the party. I've done a couple of the trips with a twilight dive just before the manta dive, and those have been above average as well. The fish seem to become quite placid and approachable at dusk, and there are often mantas around then as well.
