I usually am pretty tired after a long day of traveling so I usually relax the first day I go to Hawaii. I recommend going to Kikaua beach for many reasons. This is a beach that limits the number of cars(27) so it is very quiet. It has very fine sand as well as grass depending on what you like to lay on. If you have younger kids, they will really like the calm waters that are protected by a cove and you can have the confidence they won't be swept away by the current. There is also good sea shell gathering potential on the rocks and tide pools by the beach. If it is a weekend, make sure you arrive fairly early because of the lack of parking available, once it is full, you won't be let in.

You can also head over to Kua Bay which is very close by, there is great snorkeling as well as white sand, clean and clear water, and a beautiful setting.