Summer Streets Saturdays where from 7am-1pm you can take to the avenue by foot or bike and experience something rare in NYC: no traffic! Well, that is car traffic.
I did Summer Streets last year and things can get a little congested later in the day as it's a very popular event. I'd encourage you to get up early and take advantage of the lack of crowds while they last. All along the 7 mile route through the heart of Manhattan, there are going to be free bike rentals, free yoga classes and free performances from the NY Intl. Fringe Festival. It's a great chance to see the city in a whole new way!