The Public Gardens are a gorgeous walk at any time of year-- but during the Fall it should be particularly so! Take a break and walk from Copley Square-- it's not too bad!
If you're going to the Public Gardens, you might as well keep walking into the Commons! From there you can walk to State Street and see the State House!
Newbury Street is a relatively walkable distance from Copley Square-- some of the best shopping in Boston on this street!