For my 30th birthday a group of friends and I walked the perimeter of Manhattan island. Except for a section along the Harlem river, there are paths along the water the entire way so it required almost no navigation. We picked up and dropped off friends along way that only wanted to do parts of the walk. And we stopped at several bars too, meeting other friends that just wanted to have a drink. We started at 8am and finished just after 4am. I would definitely do it again, even without the friends or bars - but they definitely made it a memorable adventure.