Monmouth Street Park is a tiny playground right next door to the Brookline Arts Center. It consists of a single fire truck-themed play structure, a grassy area,
and a low stone wall for walking on (probably not its official purpose).
Despite its small size, the fire truck structure was a huge hit with all my
kids, from the 6-year-old to the 2-year-old – but after less than half an hour, they were done. Best to bring chalk, jump rope, balls, etc. if you're planning a long visit.