I wrote a thread about climbing the volcano Haleakala, I"m sure it can be easily found using the search function. I went with GoCyclingMaui, and would recommend that if you decide to go with this business, to do so by contacting them in advance by phone, not email. Also, I would take your pedals, shoes, saddle, and all your clothes. They use aluminum bikes, but they are the ones that have supported rides, including one up the volcano (but the one up the volcano depends on if enough people sign up, otherwise they don't do it). They are on the island of Oahu, in the town of Paia (their shop is literally at the start of the road that goes up the volcano).

Also, be prepared for the weather change. I went there a few weeks ago, and it was slightly humid (to me, at least), and the weather was warm, in the 80s. It was cloudy half the time, but the thing about Hawaii (or at least, Oahu) is that the rain is warm rain, in which it rains for a little bit, but because of the temperature, the rain stops/evaporates so quickly that, well, it doesn't really mean anything in terms of interrupting your plans.