Get brunch in the South End, or go to the SoWa for food trucks. For brunch in the South End, Coppa is delicious, but super super expensive for what it is. I really like Metropolis for brunch. If you want to do a not-cheap-but-cheaper coffee and breakfast sandwich, you could go to Render on Columbus and Mass Ave.
Then just walk around! Seriously.
Before winter sets in, the Tuesday and Friday farmer's market at Copley Square offers a bounty of local produce. In the fall, expect outrageous quantities of apples, cider, and cider donuts.
First, go to Copley Square and walk around there.
Then walk down Newbury, and you could make a stop at the Thinking Cup for coffee or a pastry or sandwich.
From Newbury, where there are a lot of shops, walk over to the Esplanade along the Charles. There's an access point on the Mass Ave. bridge.
Walk down the Esplanade to Beacon Hill.
Walk through Beacon Hill to Boston Common.
Maybe end in the North End and get dinner there.