Start your day at Block 11 Cafe (outside seating), The Neighborhood Restaurant (if you can get there early enough to avoid the wait), or Union Square Donuts (before they sell out of the brown butter hazelnut crunch!), all on Bow St., parallel to Somerville Ave. in Union Square. Afterwards, head to Prospect Hill Park, where Prospect Tower stands, only a couple blocks up Stone Ave. Climb the steps to the tower and enjoy sweeping views of Cambridge and the Boston skyline. (Warning: the park is small, but if you're in the area already, you should stop by.)
If you're still up for more walking, head to Harvard Square (make sure to pass through Harvard Yard on the way) and either stop for a sandwich at Biscuit on the Corner of Washington and Beacon St. on the way, or grab lunch in the Square -- Tory Row has large windows for taking in the Cambridge hoopla with wine, etc. on offer. Either busy yourself in Harvard Square (walk along the Charles River, shop, etc.) or take the T downtown to Beacon Hill -- get off at Charles/MGH, which is only three stops away. Check out the Liberty Hotel, then explore Charles St. and its surrounding quaint side streets and then visit the Boston Commons and Gardens where Charles St. dead-ends. If you still have energy, cross the park to walk down classy Newbury St...see what else is yours for the taking!