I spent 3 weeks in Hawaii in May. My time was split between Kauai and the Big Island. Kauai is definitely less crowded and touristy, although there are some bigger resorts/areas where you'll find a lot of people (especially the Poipu beach area -- beautiful but lots of condos, hotels, and restaurants). Still, with some research you'll find there are several isolated beaches and quieter resorts. The purpose of my vacation was more for active travel than for romance, but I don't think you'll go wrong with Kauai. Plus I found everyone to be very friendly and helpful. Definitely plan to rent a car to get around the island.

The least-developed island has to be tiny Lanai, which certainly ups the romance factor -- and then some. Check out this article, "Get Some Wows With Your Vows" (http://away.com/tripideas/outside-313664.html), plus Away.com's guide to Lanai Beach Vacations (http://vacation.away.com/lanai/travel-v2-cid283657-lanai-cid1606-beach-vacations.html).