I would recommend splitting your time between 2 islands as they are all different. The only busy spot is Waikiki where 90% of the people are, but its still a cool place to be. From there you can drive the island - the North Shore, the Dole Plantation, the Honolulu flea market are all cool things to see, I love Oahu.

Maui is great too - less crowded, a little more nature oriented. Great driving spots to haleakula, the road to Hana etc. Wailmea, Lahaina, Kaanapali are great spots to stay there.

The big island is awesome, very few people there and a lot of volcanic activity. Very cool to drive up Mauna Kea and see the view from by the observatories. I've stayed in the Kona and Waikaloa areas and loved it. For the most part the landscape here is more open, less tropical. Its a big island, takes most of the day to drive around it.

Kauai is the most rural of the 4 most visited islands. The most rural of them, the most tropical. The best if you really want to be out in nature.