My super-duper Swiss army knife with built in tracker and water purifier. ;D
But seriously, Since I guess phones and GPS is kind of cheating, probably my swiss army knife. Its got a saw, its got a blade, its got scissors... the list goes on.
It's a laser forum, the option need to be there. ;D
Really there are no rules, as long as it's just one thing. Anything humerus is welcome, or super-internet-serious-businesses if you actually want to survive.
I'd take a gazillion ton of cement and dump it in the water...
For all these people that are asking to be able to have a plane or a helicopter on the island. I assume they KNOW how to fly or do you need a PILOT also ? Maybe flapping their arms :-/ :-? :