Most importantly, get the hawaii for dummies book, it helps sooo much
Maui - Hana Highway (only go if you have the book, cause the road itself sucks, you have to know where to stop), Haleakala National Park
Oahu - Drive to North Shore, hit up the Pearl Harbor memorial its pretty sweet, and just do general sightseeing
if you can get the book, you can lay yourself out some cool stuff. And make sure to drive around the islands, and pick random roads. Its the best part of the islands, you will find great beauty by driving around, and you will soon find the sweet spots