First of all, Maui is an island, not a city. You are ON an island, not IN it.
Secondly, it's not MT. Haleakala. Just Haleakala.
Third, you don't have to do everything you possibly can in just a few days.
Just a suggestion about traffic on Maui: tourist drivers are ***holes that don't give a ***k about anybody else when they are in their g** d*** convert 'Stangs on vacation. If you plan a ride up H, go VERY early in the morning, as in you are riding through Pukalani at daybreak. Theres alot of switchbacks, twists and turns going up, many of which are blind to drivers. NOBODY is expecting to see a cyclist going up. You'll se alot of traffic coming down, because alot people go up in the middle of the night to watch the sun rise. And by 9 or 10 am, there will be a steady stream of ****ing ***k s****** going up, either hungover or half drunk.
If you rent a bike there, check it thoroughly first. It may not be in great condition.
There are other much better places to ride a bike, and much better things to do on Maui than go for a bike ride. Go to the beach, swim 200 yards past the reef, splash around alot, and be shark bait! That way you only annoying to the people at the hospital, and not the drivers in a hurry to do too much on their vacation.