Over a century ago, legendary landscape architect Frederick Law Olmstead gave Beantown a gift of green: the Emerald Necklace, a 1,000-acre, 5-mile string of downtown parks. Now, the Big Dig, a massive highway project, has done Olmstead one better, doubling the city's greenway acreage. The result? Getting your heart pumping within city limits is hardly revolutionary.
Only in a college town could a walk in the woods feel scholarly. Bequeathed in 1872 by a whaling merchant, 265-acre Arnold Arboretum functions as a living research facility for Harvard botanists. Hike a 4-mile path past two waterfalls and skyline views. More than 7,000 plants are tagged; you'll find woody plants over 100 years old; 350 lilacs that bloom in May; bonsai; even a giant sequoia. www.arboretum.harvard.edu
Skip the cobblestone tour and pedal 11 miles along the Minuteman Bikeway, a rail-trail that cuts through the cradle of the independence movement. Start cranking in Cambridge, shoot by Arlington's 183-acre Great Meadows, and pass beaver-filled Tophet Swamp, then hop the bike-friendly T back to town. Rentals at The Bike Stop (781-646-7867) cost $14 for a half-day.
Slide your kayak into the wide, sluggish Charles River near Harvard Square and paddle under the low-slung stone bridges between MIT and the tree-lined Esplanade. Herring make their way upstream through fish ladders beneath the stark new white suspension bridge come spring-and striped bass show up to feast on them. Rent a boat ($14 an hour) from Charles River Canoe & Kayak (www.paddleboston.com).