Kalihiwai Beach, Kauai
Kalihiwai Beach on the north shore, between Kilauea and Princeville, is an undiscovered-by-tourists local beach. The beach in summer is generally calm, and in the winter (or if you are traveling with small children) there is a small river emptying into the ocean that you can swim in. There are lots of trees for shade and plenty of wide open beach space for sun. You will usually run into a few locals barbecuing on the beach. A peaceful and beautiful place to spend an entire relaxed day.
It's called Kalihiwai beach, unvisited by most tourists on Kauai. Take Lihue Highway north from Kilauea a short bit. Watch for the Kalihiwai street sign BEFORE (south of) the beautiful suspension bridge. There's another Kalihiwai Rd. just north of the bridge; that's the wrong one!
Take the road all the way down to a quiet little residential street with man-made signs that say " Slow, keikis (kids) at play. " On your right, across the "street" from some funky homes, is the beach. It's big and beautiful, and the waves are rough(so watch it) but fun to play around in! Note the cliff on your right, behind you. See photo.