Honeymooned in Hawaii and been back 3 times. Short version if you plan to spend the entire trip on 1 island, make it Maui. If you want to island hop plan on spending at least 3-4 nights at each stop, except Honolulu. 2 nights is plenty there.
I'll go in order visited:
Molokai, We loved the now closed Molokai Ranch. We loved the drive and beach in Halawa and the other "secret" beaches ariund the island. The hike down (and back up) to Kalaupapa was tough but the scenery was outstanding. Papohaku Beach Park is about the largest, sparsely populated public beach I've ever seen. We laughed at the Phallic Rock and generally enjoyed the friendly chilled vibe, though I hear the locals may not be as welcoming these days.
Hawaii, the big island, there are four main parts to the island. Volcanoes NP, Kona, Waikoloa, and Hilo. Skip Hilo unless it is geographically convenient.
On our first visit we did a 12 hr circumnavigation, that included stops at the black sand beach at Pohoiki, VNP/Kilauea Crater, the Southernmost Point, (skipped the hike to the green sand beach for time purposes) and stopped in Kona for dinner before completing the loop back to Waikoloa. Other activiites included a helicopter tour, snorkeling at Captain Cooks monument, a lava hike, and visits to other area beaches.
Oahu, Honolulu is a major city so if you are not going here for city stuff, skip it. Unless you have your heart set on a sunset picture in front of Diamond Head. There are plenty of attractions though. Pearl Harbor is a must see if you are here, go as early as possible. I recommend adding a guided tour of the Missouri. Sunset dinner at Duke's with Hula Pie for dessert doesn't suck either. Outside the city, Lanikai Beach is gorgeous and the North Shore is fun to watch if the surf is up.
Kauai, very quiet but more variety and amenities that Molokai but far less than Maui. We like Poipu Beach, a good spot for a surf lesson, better than Waikiki or just about anywhere else. Some prefer Princeville, Poipu is more central. The scenery is unique and beautiful. Very lush, even the interior has a lot of scenic overlooks, including Waimea Canyon, the drive to Kee Beach at the end of the road for sunset is also a highlight.
Maui, this has everything. Lots of beaches, most all are good, just pick one closer to where you think you'll be. Kaanapapli has the most resorts and is the most developed, Kehai is bit less crowded but further from everything else. The north shore is less touristy and rougher surf but closer to the Road to Hana, (great scenery but start early or get caught in traffic) Shopping in Lahinia, plenty of water sports and boat tours out of here too. I recommend the sunrise bike tour of Haleakala if you're physically up for it, it's all downhill. Buy the book Maui Revealed, has more info/tips than anyone could want.
Whatever you do eat a lot of Shave Ice and try a plate lunch.