One of my favorite things to do in the summer is to find a neighborhood street fair. I love to see all the different wares for sale from all over the world, enjoy the street fair food, and also watch people as they do a little shopping. This fair was on 52nd Street between 7th and 5th Avenue in Midtown but they hold them all over the city.
Although I have never found one definitive guide I usually check Mort and Rays Productions, Mardi Gras Productions, or the NY Post Weekender Section when I am in the mood for a little outdoor fun.