A fire pit.

It dawns on you after thinking, it would suck to be on an island all alone in the dark. You needed a fire pit to cook food and give off light at night time. You scramble around the beach, picking up rocks that would make a nice big ring for the pit. One by one you lug big rocks over towards your tent. After getting five or six big ones, you sit down to rest. "Phew this is a lot of work..." You say to yourself. You then begin to assemble them into a ring. You clear the grass out of the center of the ring, leaving nice, rich soil in the center. 'This island is nice from the looks of it, it would be perfect to have a house on here or something.' you think to yourself. "All done. Now for some wood..." you say as you stand up and walk off into the bushes, searching for some nice wood for your fire.