Brooklyn Museum

Specifically, the third floor atrium. If you live close enough to make a membership worth your money, you can bring your child here when it's over 90 degrees and you simply cannot have them in your apartment anymore but playgrounds are hot enough to burn your skin (plus are not in the shade). The AC is elite and the atrium is basically an enormous empty room plus a few steps to make toddlers feel like they are climbing something. There are almost never people in here who are not also using the space for resting. Your toddler cannot get hurt, isn't annoying anyone, and gets some jitters out.