Rental cars are dirt cheap on the islands.... I got a convertible for under 275 for a week. Best snorkeling on Maui Is at Honolua Bay. Any local shop can give u directions. Black rock is decent too and is easy to get to. Kahana, Kannapali, Lahina is the area to stay on the west side. Road to Hana Is fun, make a whole day for it though. Haleakala is amazing at sunrise, leave your hotel at like 3am, its worth it.
I may have dismissed Oahu quickly, but all in all I didn't like it or big island nearly as much as maui or Kauai. Lanai and Molokai are still on the list to visit next. Have fun. Aloha!