Haleakula is a big deal. It is a 10,000' foot mountain you can drive to the top of. You can see Hawai'i's peaks from there. You drive through a couple really cool microclimates. You see a dormant volcanic crater that is miles wide. You get great views of the valley. And you can even see winter weather. We saw snowflakes on June 2nd. Which also means, bring a pair of jeans and a light jacket with you. And if you are on east coast time, it is so easy to get up at 3am (sleeping late for east coast time) to see the sunrise. It is pretty cool as there are just clouds on the horizon and no air pollution. Get there early and get a near astronomically dark view of the stars. And there will be at least one other hotel vistor with a beard walking around wrapped in their hotel linens that looks like Jesus at 6am.
Hana is more of a mile post than a destination on the "Road to Hana". What you really want to check out is the Keanae Penisula. When you come down to the water and the beach, walk back to that last turn in. There is a cut in the volcanic rock that the water runs up that is awesome. Then follow the road to the other side. Heavy onshore winds here make this a really cool spot for waves coming in.
Go to Waianapanapa State Park, one of the couple black sand beaches (more like black rock beaches.) There is a cave and a blowhole too.
Past Hana, you are looking for the Haneoo Rd. That beach is awesome for views too. There is an island off the shore with palm trees on it (a local story about going off to WWII). The hill there is the legendary place where Pele was buried. There is an arch across the water. Read the signs about swimming though.
I kind of think the places I listed are better than the Oheo Gulch, but it is kind of cool. You will learn is not "seven" pools nor was it "sacred pools". It can be a killer (a 12 yo girl died there while we were there in June).
The Road to Hana as for dangerous? Compared to I-95. It is like any mountain road worth going on here in the states, but without the 100mph motorcycles. It is definitely better than Irish or Scottish single tracks. Past Hana near Oheo Gulch, it gets in to give some, get some, slow around corners because there might be a bus.
If you go on, don't get in an accident as violating your rental car contract means no coverage. We went on, but that is because we rented from Kihei Rent a Car. We rented a bucket of bolts Jeep because the rent allowing you to go anywhere. But the cars are 2 steps above beaters. Nice folks. But if you do a Jeep there, plan on no security. All of the zip up windows are missing. We bought some cheap backpacks at KMart and carried everything we took (I took both my Garmin 60 and a Nuvi - I have programmed sites in ahead of time so I could find them). Google Streetview drove the southern route to Hana if you want to check it out. There are some very single lane sections. But, you can save that for your next trip to Maui But I loved it. There is another black sand beach, killer mini-canyons, big vistas, and major wind.
Iao Valley is a must. It is an easy walk in a tropical setting with rapids in the river. They started charging for the parking lot, so you will see cars on the side before you get there. Not too much. Previously, you had to do a few laps around the lot to find a parking space. You get what you pay for.
A note on KMart. We went chasing the farmers markets around. Forget it. Just go to the one in the KMart parking lot. The Swap Meet in Kahului is worth it if you have extra time, but it is not a must. The one near Lahaina used to be really good for cheap (price and quality) t-shirts. You can get all your Christmas stuff there.