Another idea would be to "volunteer" at a national forest/state park campground as a host. Every jurisdiction is different, but generally a free site for X number of hours a week.
Old post, but I just saw it (and I think I'm still within the 4 year planning window) We did almost 3 weeks a few years ago. Here are the highlights/suggestions.Minimize time on Oahu, Do the Arizona memorial and any other "tourist" spot you may be interested in and get the heck off the island. It's a big city with lots of traffic!! If/when we go back Oahu is off the list of places to go. Been there, done that! Maximize time on the big island and Maui Look into Vacation Rentals by Owner for places to stay Save your money and spend it on flying Business/First class - It's a long flight!! Plan your stays on different islands and book inter-island flights early. For our trip we booked all air about 6 months early. We flew into Oahu, spent 4 days jumped over to the Big Island (it really is a BIG island) for a week, then on to Maui for another week. The flight home was out of Maui. Each inter island flight was about $40 per person because we booked early. The closer to the flight day the more expensive. Costco in the islands sell liquor!!!! We had a VRBO house on the big island right on the water. First stop out of the airport was Costco for steaks & drinks. Held us for the week. I probably have never been so scared inmy life as the road to Hana :o