As the weather warms, think of outdoor architectural spaces. The year old green roof at Lincoln Center is a blast for kids, if slightly jarring for their parents. The roof is a hyperbolic paraboloid, which means two of the corners sort of drop off into space. The glass enclosure provides no visual reassurance that your kid isn't going to plunge on to 65th Street, but it's pretty exciting! Running around the pool below is good fun too as is the fantastic water fountain show around the corner. Very refreshing in the really hot weather!