I always spend a few days snorkeling in Honaunau Bay (AKA Two Step) whenever I am on the Big Island. I love the snorkeling here and I think it is one of the best spots in Hawaii. The coral is healthy, and the fish and other marine life varied and abundant.
The rocks on the shore get a little crowded with people, and a few tourist snorkeling boats come in the morning, but if you just get our in the water you don't really notice all that. Parking is limited and there are no amenities, although you can pay to park at the historic park and use their facilities.
This year, my wife and I swam with a pod of Spinner Dolphins for almost two hours out in the bay. I like snorkeling on the South side of the bay, past the freshwater springs and out along the City of Refuge. There are many lava ridges to explore which are covered with coral and marine life. There are a couple of caves that you can swim through too, if you are brave.
Just north of the bay is Arch City -- amazing blow holes and sea arches that go on and on as you hike north along the coast. There are a couple of primitive trails leading to Arch City at the north end of the one-way road at Honaunau Bay (just before the junction with the highway).