The longest beach in the entire state of Hawaii is three-mile-long Papohaku Beach on the west end of the island of Molokai. I didn’t believe the guidebook, but it was right about how deserted this would be. It was Sunday afternoon at 2 PM, but ours was the only car in the parking lot. There was only one other couple on the entire beach, and they were over a mile away from us. (They must have parked somewhere along the road, since their car was not in the lot.) This is a beautiful empty endless expanse of wide light-colored sand, and you can have it all to yourself!
If you're going to Maui as well as Molokai, take a look at the great information on this website:
www.mauihawaii.org