Boss Frog's on S. Kihei road is probably closest to you for rental. I rented a Fuji Altamira from them just last week.

And for rides, Mt. Haleakala is THE ride to do. Not downhill, that's easy, but uphill. I tried and failed. See my post here on the forum.

I'd heard that the west Maui loop is actually difficult, with very narrow roads and some dangerous cliffs. I guess clockwise doesn't put you against the cliffs though.

Have fun. I sure did.