The big attraction on The Big Island of Hawaii is the Volcanoes, watching the crater as the sun goes down over Kilauea is rather special, the only thing you are not prepared for is the darkness, complete darkness so when you do eventually draw yourself away and walk back to the car park a torch would be a good idea, there are no lights at all so with a lot of people blundering around finding your car is quite interesting. https://www.nps.gov/havo/index.htm
The Hawaii Tropical Botanical Gardens http://www.htbg.com/ a short way north of Hilo are the nearest I have been to The Garden of Eden, maybe I was lucky to visit early on a Sunday morning when it was very quiet. As you walk down into the gardens the sound of birds and insects is everywhere, each corner you turn there is something special to see, as you get deeper into the gardens you start to become aware of the sound of the Ocean, getting louder and replacing the bird song, until suddenly you are on the cliff side, the water rushing into inlets either side of the garden and the Pacific stretching away into the distance, if ever there was a place to visit for tranquility this is it, highly recommended.
Back on Maui we drove up to the summit of Haleakala https://www.nps.gov/hale/index.htm a dormant volcano over 3000m in height, but still over a 1000m short of Mauna Kea on The Big Island.
The views are spectacular, you can clearly see Mauna Kea which was covered in fresh snow when we were there, the clouds move quite quickly, so one minute you can see nothing and then suddenly amazing views like above appear, the park is open 24 hours a day, so it is very popular to go and see the sunrise, that is something I must do!