Now that is a hard question, "what is the coolest thing to do on PEI?" Now age does come into this question, hanging out at Basin Head Beach is cool for the younger crowd; Kings Castle Park is cool if you want to relax with the kids.
Doing a craft beer tour can be very cool. (With a designated driver of course).
Clam digging and cooking them yourself is really cool if you like the outdoor adventure.
There are lots of cool things around here, come on down and fill up!