Adding seating outside of our larger parks is already don, here's a view of this at Central Park
|And another out of the park Seating at Madison Square Park|
Here's West 77th Street and Columbus Ave, There is a schoolyard playground here and nothing on the outside except a street for circulation.
By the addition of seating and plantings the sidewalk was activated and transformed into a sidewalk park.
By doing a similar transformation to NYC playgrounds, it can resolve the issue stated at the beginning of this post.
Additionally the parks dept can create street paklets with active recreation for adults in place of parking in front of playgrounds.