It seems you are asking for a lot of info in a short period of time. You should have done your homework weeks ago. There is a thing called Google. Every island is different, so helicopters flights are different. Just did a flight out of Kona over the volcanoe Nov. 29 and saw spectacular eruptions and lava flows. Understand two days later , nothing. If you book your own flights and for some reason you don't make the port, you are responsible for the cost.
Car rentals, which island do you want to drive around?
Maui has great drives and beaches for snorkeling. Time permitting you can drive up to the crater at 10,000+ feet, or go to Hana and swim in waterfall pools or snorkel. Helicopter flights are spectacular over the crater and valleys.
The big island, Kona or Hilo. You can drive to black sand beaches, waterfalls, Volcanoe National Park to see the craters, lava flows. Great snorkeling in Kona. Helicopter flights can be outstanding if the crater is active. You can fly by the highest waterfall in the islands.
Oahu has many great sights. Drive to the north shore and maybe see some big surf at the Pipe Line or Sunset Beach. Visit the beaches and sites where the TV series LOST are filmed. Great snorkeling at Hanamau Bay. A national treasure. The Arizona Memorial and Pearl Harbor, The Punch Bowl, very memorable experiences.
Kauai, the Garden Island. Helicopter flights over the Grand Canyon of the Pacific are overwhemling. Drive around the island, and you can find great snorkeling or swimming beaches. Poipu beach is world renowned for snorkeling and swimming.
You can rent snorkel gear at every port or city. I don't want to sound to critical, but I don't think you were very well prepared.
I would suggest picking up the guidebooks Maui Revealed, Kauai Revealed, etc. They have great info on various places for snorkelling. I was able to pick up copies at the local library. ***edited to remove commercial reference***
We booked our helicopter trip on Hawaii with Safari Air. They told us that if the ship doesn't make it into port, they will not charge us. Many of the others it was totally non-refundable within 24 hours. We LOVED the helicopter trip and wished we had scheduled another trip for Kauai!