Time your trip to coincide with the five-day Sail Boston Festival/Tall Ships Atlantic Challenge (July 8-13;) festival with parades, harbor tours, and New England cuisine. If you miss it, there's still fifteen acres to ramble on on the The Rose Kennedy Greenway. The public space welcomed spring with 10,000 daffodils, but there's also modern sculpture, light installations, and flourishing parks and gardens.
Stroll through four centuries of Boston history on the Walk to the Sea. Start at the State House and end a mile later at the Long Wharf, a segue to the newly completed HarborWalk. Each waterfront property maintains its own portion of the interconnected pathway, lined with small parks, viewing areas, and benches. Get a view from above on the fourteenth floor of the Independence Wharf Building (470 Atlantic Avenue); just sign in at the front desk and take the elevator to the public patio.
You had the poster, now see Shepard Fairey's retrospective (through mid-August) at the Institute of Contemporary Art building, which opened in 2007 and practically hangs over the water.
Buy local at Achilles Project in Fort Point Channel, which includes a fashion boutique that carries exclusive designers; a restaurant, Persephone, run by a five-time James Beard-nominated chef; and a local art gallery and event space.