We spent most of our time on the beaches on the north side of the island, so these are the only ones we can report on. We went to Kalihiwai, Anini, Hanalei, and Ke'e beaches.
Our favorite beach was Hanalei, because we had the most fun there. We took surfing lessons, which we loved. The beach is nice and long, great for walks, and the slope of the beach is very slight, making it a great place to learn to surf. You could walk out to where the waves were breaking and still have good ways to go back in to catch the wave. The sand at this beach is amazingly soft--the softest that we experienced. But it's also very hard to get out of your swimsuit! We didn't see any coral around, just nice, soft sand for the waves to roll in on. Our kids, 2 and 4 years old, had a great time in the waves.
Another beautiful one is Ke'e Beach. It's at the end of the road, on the north side. My husband and I went early and did the hike along the Napali coast, 2 miles in to this rocky beach. It was well worth the effort for the gorgeous scenery and much-needed exercise! We then met the kids and grandparents for lunch on the beach. The beach here is much different. There's lots of coral, so it's great for snorkeling. There's a nice protected little area to swim around in without having waves come crashing in. And we saw a few fish.
My husband loved Kalihiwai Beach because of the waves for boogy-boarding. We found this beach looking for Anini and loved it. It's at the end of Kalihiwai road, where the bridge was wiped out by a recent tsunami. We went on a weekday, and there were only about six other people there. It's not a very big beach. Kids had fun building sand castles but were kind of afraid of the waves. They came in kind of fast.
Anini Beach was nice but my least favorite. There were a lot more people there when we went. The water is pretty calm and shallow, but there were lots of rocks in it. We saw a couple of parks with restrooms and several tents set up for camping.