This surely is the best thing you can do while you are on a beach. You can start a treasure hunt with your family member or friends. Collect printed scavenger hunts and hide it everywhere around you. Then make your friend look after it.
If you are not fond of printed cards for a treasure hunt, you can also hide kinds of stuff like beach toys, sunglasses, small shells, or sand dollars. This game can be made more fun when you set time limit and a wired punishment.
You must agree this is the best time-pass but younger people might have got bored with this hunting game. In this case, this game can be very useful when you are busy doing works and want your kids to be engaged in less dangerous stuff.