I'm going back to Molokai during my honeymoon, which will also include stays in Kauai and Lanai.
I can't recommend Molokai highly enough. It's a genuine, down-to-earth place, with a variety of outdoorsy stuff to do.
Go out to the West Side and you can be on a humongous beach all by yourself. Take a short walk up to Phallic Rock (yes, the name is more or less on target), take the narrow road to Halawa Falls and sit by the water or swim with the locals. I haven't done the mule ride yet down to the former leper colony, but I've heard it's worth it. I'm pondering the cost, for the moment (about $150).
You can mail a coconut instead of a postcard from one of the post offices (not the one in Kaunakakkai).
Overall, it's a place to relax and imagine how all of Hawaii was before it was discovered by Jack Lord.
Have a great time.