Open up the wallet of a Bostonian, and you're likely to find a Charlie Card  hidden amongst Red Sox ticket stubs and Dunkin Donuts receipts. A must-have for any city dweller, a Charlie Card unlocks hundreds of things to do in the area by granting holders access to the subway, better known as the T. The red line deposits passengers right outside Boston Common, a nearly 50-acre park with blossoming trees and grassy fields speckled with sunbathing college students. During the wintertime, families flock to the Boston Common Frog Pond to ice skate and drink cocoa in the crisp evening air.

The Common is the starting point of the Freedom Trail, a 2.5-mile walking tour that takes sightseers to 16 local landmarks and historical sites. Tour groups meander down a red-brick path, admiring the golden dome of the centuries-old Massachusetts State House before examining the tombstones of the King's Chapel Burying Ground, the oldest cemetery in Boston. After perusing antique firearms inside [Faneuil Hall, trail goers can pop into the nearby Quincy Market Colonnade to sample international eats from more than 40 vendors.

Hop off the green line at Science Park station for a visit to the Museum of Science, a treasure trove of more than 600 exhibits and activities. Visitors can take a simulated flight to the moon in a full-size space shuttle before gaping at dazzling electrical bolts in the world's largest lightning generator. In the museum's Mugar Omni Theater, audiences marvel at the images of plants, animals, and far-off lands that flicker across a five-story screen.

Located right on the Boston Harbor, the New England Aquarium is home to more than 8,000 aquatic species. Little blue penguins romp around an open pool, and blubbery creatures sunbathe on the rocky outcrops of the harbor seal exhibit. A staircase winds around a three-story saltwater tank teeming with colorful tropical fishes, giant turtles, and grinning sharks. From April through October, the aquarium offers whale-watching trips, during which passengers sail to the feeding grounds of whales, dolphins, and seabirds.

The roar of cheering crowds echoes across downtown during baseball season, when fans flock to the historic Fenway Park to swill on pricey beers and root for the Red Sox. On game days, the streets around the park come alive with sausage carts, live music, and vendors hawking Boston gear. If you don't manage to score tickets, you can still visit the century-old stadium for a guided tour of the warning track, press box, and Red Sox Hall of Fame.

To explore Boston's college culture, take the green line over to Allston. In this lively student neighborhood, rows of fraternity houses are broken up by vintage shops and pizza joints. Boston University students chatter over mountains of nachos at Sunset Grill & Tap, sipping on selections from its more than 110 draft beers. Hipsters crowd Silhouette Lounge, where the bartenders are friendly and the popcorn is free.

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