Going to the outer islands will be your best bet. The big island offers a lot of variety. I've done the Ironman there and it gets windy on the Kona coast, if looking at lava fields and riding in heat and wind appeal to you its unparalleled. The Hilo side offers a more lush landscape.
Maui has a beautiful ride out to Hana where you can find some great camping or perhaps a serious ride up to Haleakala National Park (10,000 feet) where you can camp with a permit and have an unforgettable descent down.