Home to MIT, Harvard, and Tufts, Cambridge and Somerville host a large, diverse population. Each area has its own special "feel" - the hip and trendy Davis and Inman Squares, biotech-centered Kendall Square, or eclectic Central Square amongst others.
There are great ways to get exercise along the Charles River. Areas of Memorial Drive are closed to cars on Sundays so pedestrians and bikers can enjoy the river. There are boathouses at Kendall Square and the Charles River basin to rent kayaks, canoes, or sailboats. If you enjoy biking, ride the Minuteman Bikeway to Lexington and beyond. Runners have a multitude of amazing possibilities (running map).
Cambridge and Somerville host hundreds of excellent restaurants. Gastro-pubs are scattered all over, but try Green Street Grill in Central, or Russell House Tavern in Harvard, amongst a multitude of others. Visit Julia Child's favorite Chinese restaurant at Mary Chung's, or some authentic Taiwanese food at Mulan, both near Central. Or try the Japanese Market in Leslie University at Porter, with several small stalls. Cambridge also has excellent Indian, Thai and other ethnic cuisines.
For late night entertainment, clubs include Middlesex in Central. For those who love salsa or swing, try the Havana Club in Central. For movies, visit the Brattle Street Theater. For plays and theater, try the American Repertory Theater. If you like pool, try Diesel Café in Davis.
There are numerous performances during the school year in all 3 undergrad campuses. Also visit the Harvard museums - it is amazing how much art and natural history is hidden in the back halls of the University!