Cemeteries and Arboretums are often dog free.

In particular, try Mt Auburn cemetery which is incredibly beautiful in the spring and fall (or with snow, I imagine)

Also Appleton Farms: Appleton Farms (978) 356-5728 https://maps.app.goo.gl/hpbXZS3QittvNVaaA?g_st=ic

The Trustees of the Reservation have a number of other sites that are dog free.

Here’s another example: https://thetrustees.org/place/boston-community-gardens/