Arnold Arboretum is Boston's great tree collection: 281 acres of rolling paths, ponds, meadows, and living plant collections managed by Harvard but open as part of the city's park system. You can treat it as a serious botanical garden or simply come for a long walk under maples, conifers, lilacs, and old specimen trees.
The bonsai collection is a highlight, with some trees more than two centuries old. Picnics are tightly limited, but walking is easy and the arboretum pairs well with Jamaica Plain or Forest Hills Cemetery.