City-center Boston Common is the oldest public park in the country. Cattle grazed here between 1634 and 1830, but today, its rolling green hills and bench-lined paths are populated with sunning and strolling Bostonians of every stripe. Kids frolic in the fountain of the shallow Frog Pond during the summer; in winter it becomes a picturesque ice-skating rink. The adjacent Public Garden (www.friendsofthepublicgarden.org) is more formally landscaped, with flower plantings, a statue of George Washington on horseback, and a pond where the famous pedal-powered Swan Boats operate from mid-April through mid-September (www.swanboats.com). A beloved bronze sculpture of baby ducks on parade is in the northeast corner of the park, inspired by Robert McCloskey's children's classic, Make Way for Ducklings. Both parks are good places for a picnic lunch. Cute sandwich shops line Charles Street; local favorite, Finagle-a-Bagel, is directly across from Boston Common (129 Tremont St.; 617-426-3300; www.finagleabagel.com); and Chacarero, in Downtown Crossing, serves up Chilean sandwiches of chicken or beef with Muenster cheese, string beans, tomato, avocado spread, and hot sauce (426 Washington St.; 617-542-0392; www.chacarero.com; closed weekends).

Just a seven-mile ferryboat ride across the Boston Harbor, this little-known national park makes a great day trip on a sunny summer day. The park extends to 34 islands, but only six are accessible to visitors. Fort Warren on Georges Island was built in 1833, and served as a military training ground and a Civil War prison. Nature lovers will enjoy Grape Island, which has a multitude of shorebirds and berry bushes. Ferries depart from Long Wharf in front of Christopher Columbus Park and run between Georges, Lovells, and Spectacle islands; interisland ferries shuttle to Grape, Bumpkin, and Peddocks islands. The schedules vary depending on the season and the day of the week; be sure to check the schedule online in advance). And pack a picnic lunch, as food options on the islands are slim (just hot dogs, subs, and sodas on Spectacle and Georges).

Early May–early October only.

Even Yankees fans agree that there's something special about seeing a ball game at Fenway Park. First opened in 1912, it's one of the smallest stadiums in the major leagues, and it's always packed (every game has been sold out since May 15, 2003) with fans eagerly awaiting a home-team hit over the Green Monster, the 37-foot-high left-field wall. The whole experience is steeped in tradition, from the manual scoreboard to the organ to the Boston-accented hecklers. Grab a sausage-and-pepper sandwich outside the park before or after. Remember, Massachusetts's liquor laws are strict; beer vendors do not wander the stands, so you'll have to go get it yourself. And bring ID, even if you haven't needed it in years. Daytime tours of the stadium are also available; see the website for more information.

Billing itself as "Boston's Left Bank," Cambridge is an academic center, a technological corridor, and a vibrant, multicultural city located just across the Charles River from Boston. It's easily reachable on the T, and you'd do well to set aside an entire day to explore it properly. In Harvard Square, street musicians compete for attention with socialists handing out literature. Purists complain that it's become too commercial and that there are too many chain restaurants and shops (and it's true, you'll find the usual Gaps and Pizzeria Unos), but it's still a great place for strolling and people-watching with an ice cream cone from Herrell's (15 Dunster St.; 617-497-2179; www.herrells.com). South of Harvard along Mass. Ave. (only tourists call it Massachusetts Avenue), Central Square is a corridor of restaurants, bars, and shops with a funkier, edgier feel, such as the Middle East music venue and the 1369 Coffee House (1369 Cambridge St.; 617-576-1369 and 757 Mass. Ave; 617-576-4600; www.1369coffeehouse.com).

At Harvard University, get your bearings at Harvard Information Center, located in the Holyoke Center arcade (1350 Mass. Ave.; 617-495-1573), then walk around Harvard Yard to admire the centuries-old academic and residential buildings. There are three art museums to choose from (www.artmuseums.harvard.edu), American and European works at the Fogg (32 Quincy St.; 617-495-9400), art from German-speaking countries of northern and central Europe at the Busch-Reisinger Museum (32 Quincy St.; 617-495-9400), and Asian, Islamic, and Indian art at the Sackler (485 Broadway.; 617-495-9400). Even non-science types will be impressed by the Harvard Museum of Natural History, home to the intricate Ware Collection of Blaschka Glass Models of Plants, commonly known as "the glass flowers." It includes over 830 species that were created as early as 1886 for botany students to study (26 Oxford St.; 617-495-3045; www.hmnh.harvard.edu). Across the street from Harvard Yard, the Sanders Theatre presents concerts (from folk to classical music) and public lectures (Jane Goodall was there in March 2007). First used in 1876, this all-wooden space evokes old English academia, and is prized for its acoustics (45 Quincy St.; 617-496-2222; www.fas.harvard.edu/~memhall/sanders.html).

Winding around 16 historical sites, the 2.5-mile Freedom Trail is a good introduction to Boston history—and also to the city's sometimes-complicated geography. Pick up a map at the visitor's center on Tremont Street at the edge of Boston Common and walk along the red line on the ground (it's sometimes painted, sometimes lined in brick). While it's possible to walk the trail in an hour or two, leave time to stop along the way. You'll pass the graves of John Hancock, Samuel Adams, and Ben Franklin's parents at the Granary Burying Ground; Boston's first meeting house, Faneuil Hall, which hosted debates about the Sugar Tax of 1764 and the Stamp Act of 1765—note the distinctive grasshopper weather vane on the top of the building (4 South Market Building; 617-523-1300); and the Paul Revere House. Dating to 1680, it's the oldest building still standing in downtown Boston and a good example of Colonial-era architecture (19 North Square; 617-523-2338; www.paulreverehouse.org). As immortalized by Henry Wadsworth Longfellow in his poem, "The Midnight Ride of Paul Revere," two lanterns (as in "two if by sea") were hung in the belfry tower of the Old North Church to signal the landing of the British in 1775. It's a lovely building, though you cannot climb the tower (193 Salem St.; 671-523-6676; www.oldnorth.org). Launched in 1797, the U.S.S. Constitution is the oldest commissioned warship in the world that's still afloat. U.S. Navy sailors take you below deck to explain what life was like for early-19th-century seamen (1 Constitution Rd.; 617-242-7511; www.ussconstitution.navy.mil).

Guided tours are available daily between April and mid-November. Specialty tours, such as a historic pub crawl, are held during the rest of the year. Tickets can be purchased online.

Isabella Stewart Gardner was an heiress and something of a black sheep of late-19th- and early-20th-century Boston society: She was a rabid Red Sox and horse-racing fan. Funded by a $1.75 million inheritance from her father (a linen merchant) and husband (a financier and heir to a shipping fortune himself), Gardner personally designed this four-story building, modeled on a Venetian palazzo, to house her extensive collection of art and antiquities. The exhibits include paintings by Manet, Sargent, Holbein, Whistler, Rembrandt, Matisse, Michelangelo, and Titian; 15th-century Flemish tapestries; a first edition of Dante's Divine Comedy; and inscrutable documents—look for the one signed by Marie Antoinette. A visit to the museum is like meandering through the attic of a wealthy, if eccentric, old aunt. Mrs. Gardner believed that art should be appreciated on its own merits, so almost nothing is labeled, though some rooms have laminated information cards that fill in the blanks. The indoor garden courtyard, filled with citrus trees, orchids, and seasonal plantings, is as impressive as the collection—and there's an explanatory book about it that you can pick up at the information desk.

Open Tuesdays through Sundays 11 am to 5 pm.

Housed in a modern building at the mouth of the Charles River with views of the Boston skyline, the Museum of Science is one of the best of its kind. Kids love the hands-on exhibits that demonstrate everything from biology to computers to physics. They can determine how much sugar is in a bottle of Coca-Cola, measure the volume of their lungs, and investigate the mysteries of lightning with the world's largest Van de Graaff generator. There's also an adorable troupe of live cotton-top tamarin monkeys who live here to help the museum staff study evolution, a planetarium, and an Omnimax theater that shows science- and technology-oriented films, such as Galapagos. Volunteers roam the building on Segways to dole out information. The Butterfly Garden, a humid greenhouse opened in 2005, flutters with about 100 butterflies (monarchs, owl butterflies, and bamboo pages) and exotic plants, like the Stachytarpheta (or Blue Rat's Tail). The place is generally mobbed by school groups on weekdays, and herds of suburban families on weekends (a huge parking deck sits next to the building to accommodate them). Visit on a Friday evening after 5 p.m. to avoid the crowds.

Open daily 9 am to 7 pm (until 9 pm on Fridays) between July 5 and Labor Day; 9 am to 5 pm (until 9 pm on Fridays) during the rest of the year.

Located right on Boston Harbor on Central Wharf, the New England Aquarium is home to animals from both the northeast region and around the world. Atlantic harbor seals greet guests from an outdoor habitat, and three of the 17 penguin species are represented here—you can use a beam of light to encourage them to play in the water, or show up at 9 a.m. or 2:30 p.m. to watch them be fed. There are exhibits on local ecosystems, such as the Boston Harbor Islands, but the centerpiece of the building is the 200,000-gallon Giant Ocean Tank. Start at the top and wind down the ramp to view Murtle the Turtle, a 550-pound Atlantic green turtle, as well as schools of fish, sharks, and rays that rub right up against the glass. The largest IMAX theater in New England, which screens 3-D movies such as Deep Sea 3D (narrated by Johnny Depp and Kate Winslet), is housed in an adjacent building. Between April and October, the aquarium hosts whale-watching tours; they last three to four hours; book in advance online.

Open daily 9 am to 6 pm.