When I went to Maui I did a bike tour/ride with Donnie Arnoult with Go Cycling Maui. He rents Litespeeds and does an awesome job of supporting his riders!! I am not affiliated whatsoever with him, he just does an awesome job and you can also rent bikes from the bike store that he owns. Here is his website:
http://www.gocyclingmaui.com/index.html
Climbing Haleakala is friggin awesome!!! Over 10,000 ft of climbing in only 38 miles!! Have a great summer!!!