I've ridden in Maui a few times, particularly over the last 18 months. I did the Hesjedahl Maui Cycling Camp in December and we literally rode all around the island (or at least the younger, faster riders did while I sagged part way).
Many roads have decent shoulders and there's a bike path across the island to avoid the main road to Kahului. West Maui loop is a highlight. Did it by car in 95 with my wife and kids counter-clockwise and my wife was hanging on for dear life. Drove it the opposite way and have ridden it twice clockwise recently. It is a really great ride with lots of little ups and downs and great views of the ocean if you dare to look while riding. Yes, much of it is pretty narrow but it's less scary on the bike than in a car. It's the one road with few places to get drinks so look at your map and stock up at what look like towns on the map. There's one shaved ice place about half way along the narrow stretch and that's about it. Our camp also rode all the way around the east Maui loop. About 20 km of really bad road with no traffic and very long (maybe 180 km if you have to do a full loop) but a stunning ride. As BoulderDad said, you can ride Paia to Hana (or part way). Much of the ride is along the coast. In the old days (I first went to Maui in 1979), the road to Hana was treacherous in a car but there's been a lot of widening. Just be really careful to stay well to the right, particularly at corners. There's also some nice little hilly rides from Paia if you're staying on that side of the island.
Haleakala is great. There are lots of accounts of that ride on the net but it's long. Also, the last place to buy drinks was closed when I was there in December and it looked permanent. There's a fairly big store in Kula that's just below the road that's probably the last place to buy drinks. You can also get water at the park office once you're inside the park. Remember your 5 bucks or you won't get into the park. I've ridden all the way in September when it was too cold to ride down (threw the bike in the support vehicle my wife had driven to follow my son and me) and to the park gate in December when a light jacket was enough even coming down for long stretches at 60 km/hr+. Most roads have lots of towns so you can usually get Gatorade or water.
I've dealt with bike shops in Lahaina, Kihei and Paia. The one in Paia also has rides (http://www.gocyclingmaui.com/
). They provided support to our camp and were great. The shop in Kehei rented Treks and I'd rate as OK at best. The one in Lahaina had Specialized and seemed a little more willing to help than Kihei. We rented bikes from both of those and the bikes were fine, although not cheap. Donnie in Paia is a nice guy and knows every road on the island after running a bike based business there for about ten years. I got to know a couple of his guys pretty well since they were designated to ride at the back of the group and they were good guys. A friend rented a bike from them a few weeks ago and it was OK too.
I'll be back there in two weeks and plan to ride the west Maui loop in both directions since there isn't much else to do from Lahaina. I'll also fiddle around up and down the coast a bit and maybe try some road in from the coast near Lahaina (all pretty short I think). We talked and talked and talked about going somewhere new or someplace warm for some time away from the cold. We didn't get our act together on anyplace else so we're back to Maui for the 13th time (9 from 85-95 when kids were little, none from 95-late 11, four times since).