Here's a great little secret I learned a few years ago: Kauai has fabulous county parks where you can camp right on the beach. And you can't beat the price: $3 per person per night. Yep, for $3 you get beachfront accomodations and lapping waves as your lullaby! My husband and I camped in three different parks in three different parts of the island. Each park had a gorgeous beach, a grassy area for tents (no sand in your sleeping bag!), barbeques, restrooms, and plenty of parking. The only drawback is no hot showers, but Hawaii is so warm that it really doesn't matter.
Our favorite campground was Salt Pond Park in southwest Kauai. The beach there offers great, safe snorkeling. And on weekends, Hawaiian families gather to grill enormous amounts of food, strum their ukeleles, and just kick back and laugh. We relished the sights, sounds, and smells of authentic island life and would chose Salt Pond over a resort any day!
You need a permit to camp in Kauai's parks. You can find all the details on this website:
And here's a photo of the beach at Salt Pond Park.