The most fun thing to do in Hawaii is hang out at the beach. One of the most accessible beaches in the Kona region is family-oriented Kiholo Bay. The bay is large with plenty of sandy beach access, protected salt water pools, and small fresh water swimming holes. Excellent fishing and diving. Sea turtles abound, curiously floating in the shallow water and munching on the reef.
Free camping spots line the beach - but a 4WD truck is necessary to get to them and the camping here is pretty much for locals only. Trucks can drive along the beach and park. To get a camping spot for the weekend, you must show up Thursday afternoon or early Friday morning.
To get to Kiholo from Kona, travel north past the airport. After Kekaha Kai State Park, look for a gravel road to the left marked by a "barber pole." There are no signs. The road is accessible to cars. Travel the road toward the beach. When the road splits, either go straight and park by the portable toilets (only enough parking for about 5 vehicles) or veer left and park by the octagonal building and other portable toilets. After you park, walk either south or north along the beach. The main bay is to the north. However, there is good swimming just to the south at the pebble covered, black sandy beach. There is no reef here to climb over, and the water gets deep fast. Make sure to swim out a few feet past the shore breakers. I've seen tourists just stand in the shallow water right where the waves are breaking. Don't do this or you will get pounded by water. Duh.
The sun radiates off the black sand and creates hot conditions - bring sunblock and slippers (flip flops). You have to wear flip flops or shoes to follow the shore to the north as the sand is super hot and composed of large pebbles which hurt bare feet. Bring a beach blanket - there are plenty of sandy areas to discover. But pack light - the walk is too far to carry a beach chair, for example.
This is a good beach for people people who like the water and those who don't. While the scenery is not necessarily lush (scrubby bushes and lots of lava flow) - there is plenty to look at and discover while strolling/exploring. The ocean is clean and clear. There are few people here (after you walk past the camping families).
It's refreshing to jump in on your way back, after a hot walk along the beach. After Keanalele water hole, continue North. Pass a couple interesting big houses and a charming, chaotic surf shanty. You will see the rest of the bay after a few curves of beach - have fun exploring.