Iao Valley in the West Maui Mountains is a beautiful State Park, although it tends to get really crowded on the weekends.
The major attraction is without any doubt Iao Needle, a rock pinnacle that rises 1200 feet from the valley floor. There are some short, paved (!) walkways in the park, from where you can get different views of Iao needle.

However...if you wanna see how Iao valley looks like, there is a small trail (unmarked), starting right behind the last viewpoint, a platform with a shelter and a low wall. The trails starts off 'on top' of that wall, leading towards the mountains. I don't know how far it actually goes (probably as far as you like), but already after half an hour you'll get a fantastic view of the valley with mountains rising on every side and waterfalls splashing down into the valley. It's defefnitely worth the short sidetrip...