When you do the Hana highway drive you might consider going on around the backway. It's a long drive, a few potholes but certainly not the impassable road the rental car companies make it out to be, we made it just fine in a regular ol rental car. Anyway it's a part of the island seldom seen or photographed. It comes out about halfway up on Haleakala.
Drive north out of Lahaina and Kaanapali. Up on the north coast there is a blowhole. I've only been in the winter and it was going both times then, but with calmer seas like you'll get in August it might not be going. Even so walk down to it, there's some wild looking rock there where the salt spray from the blowhole has corroded them over the centuries.
Haleakala sunrise can be real hit or miss. The first time I did it it was one of those life lists moment. The 2nd time the sun rose for about 10 seconds and then a cloud blew across and it was all fog. There's a hike that goes down into the crater and out at a lower elevation. Do it if you have someone that can pick you up at the exit point. Dress WARM for the sunrise, i've always froze my ass off up there until the sun gets up, especially the hands, you just don't think to bring gloves to hawaii lol.
Buy this guidebookhttp://www.wizardpub.com/maui/maui.html
It shows all kinds of spots all over the island that most people drive right past.