For Brookline, the Brookline historical society has walking guides if you’re interested in history. Even if you’re not, they do point you to interesting neighborhoods to walk in. They also have a list of all the “secret paths” at the bottom of the walking tours page and I think that’s a good way to explore Brookline. Just be prepared for a lot of stairs!! https://brooklinehistoricalsociety.org/tours/walkingTours.asp