Solo, Back Bay (Commonwealth Ave Mall) works.
Of Boston's two great residential walks, this is the grand, formal one — the Commonwealth Avenue Mall runs a wide double row of elms and statues straight down the spine of the neighborhood, flanked by uniform Victorian brownstones, and it feels less like a street than a slow processional promenade, especially under spring magnolia blossom or in deep fall color. It's the one stretch of Back Bay that earns its own visit rather than overlapping with Newbury: where Newbury (a block over) is shops-and-patios and Beacon Hill is intimate and crooked, the Comm Ave Mall is open, axial, and quietly imperial. The catch: it's still residential, so the 'activity' is the walk itself, and outside blossom or foliage season it's simply a pretty, calm green corridor.