Well I myself prefer coastlines for sunset/sunrise, unless there is some other interesting landscape to 'use' available. One important thing is where you are located on the island, it takes pretty long to drive all the way around. I have not seen that much of the eastern coast (it is not the most famous part of the island), maybe the Wailua River mouth could be an option there.
Other than that I would recommend the beaches and bays east of Poipu, e.g. Shipwreck or Maha'ulepu, the second is not so easy to reach however (dirt road which is usually not allowed with rental cars or by hike). They both face southeast and there are lots of small cliffs as observation points there, the landscape is beautiful.