If you are New York city this summer, definitely take a stroll on the NY Highline. It took me about an hour and a half to stroll the whole 3 miles that are open as of July 2011. It's fun, and best of all, it's something to do in Manhattan that is FREE.
Also, be sure to stop mid-way to walk the Chelsea Market (accessible at street level), you won't be disappointed... There is something for everyone to enjoy, from snacks to a full meal. Plus, it gets you out of the sun for a bit.