Been there; Done that!
Dan, we were just in Hawaii last year, rented planes from a couple of different places, and flew all the major islands.
Kawaii is a must, but there are no plane rentals there. We rented in Oahu (Honolulu airport) and flew Kawaii from there. A GREAT experience. The FBO we used was ?????????? and the instructor was ???????????????.
We took an instructor the 1st day, then flew the rest of the time by ourselves. I strongly recommend that, as Honolulu has defined departures and arrivals that are based on local landmarks (golf courses, highways, manufacturing plants, bays, volcanoes, etc..) and, if you are not familar with them, you will not be able to fly them. Also, because of the nature of the islands and all the sightseeing air traffic, there are special frequencies, beyond what you might be use to, that are used for air-to-air traffic. Finally, their requirements (similar to requirements elsewhere across Hawaii) are such that the alternative to taking the instructor with is just as expensive as one, and not as beneficial. Ground school, extensive practical checkout, etc..
I'm in a rush right now, but email me and I can suggest some excellent materials that you will want to secure and take with you (and study ahead of time). I think I'm set up so that you get my email. If not, just post back to here and I'll get it.
We are flatlanders. This was the first time we flew mountains and water expanses. What a thrill!! You are in for an adventure of a lifetime!