Keep in mind that the Boston Public Garden and Boston Common (not Commons!) are two separate entities with different histories and purposes and a distinct boundary between them at Charles Street. The Common has been public land since Boston was founded in 1630, whereas the Public Garden belongs to a newer Boston, occupying what had been salt marshes on the edge of the Common. By 1837 the tract was covered with an abundance of ornamental plantings donated by a group of private citizens. The area was defined in 1856 by the building of Arlington Street, and in 1860 the architect George Meacham was commissioned to plan the park.
The central feature of the Public Garden is its irregularly shaped pond, intended to appear, from any vantage point along its banks, much larger than its nearly 4 acres. The pond has been famous since 1877 for its foot-pedal-powered (by a captain) Swan Boats (swanboats.com), which make leisurely cruises during warm months.
The pond is favored by ducks and swans, and for the modest price of a few boat rides you can amuse children here for an hour or more. Near the Swan Boat dock is what has been described as the world's smallest suspension bridge, designed in 1867 to cross the pond at its narrowest point.
The Public Garden is America's oldest botanical garden, and is replete with gorgeous formal plantings. The beds along the main walkways are replanted for spring and summer. The tulips during the first two weeks of May are especially colorful, and there's a sampling of native and European tree species.
The dominant work among the park's statuary is Thomas Ball's equestrian George Washington (1869), which faces the head of Commonwealth Avenue at the Arlington Street gate. This is Washington in a triumphant pose as liberator, surveying a scene that, from where he stood with his cannons at Dorchester Heights, would have included an immense stretch of blue water. Several dozen yards to the north of Washington (to the right if you're facing Commonwealth Avenue) is the granite-and-red-marble Ether Monument, donated in 1866 by Thomas Lee to commemorate the advent of anesthesia 20 years earlier at nearby Massachusetts General Hospital (you can visit the hospital to this day and see the famous Ether Dome). Other Public Garden monuments include statues of the Unitarian preacher and transcendentalist William Ellery Channing, at the corner opposite his Arlington Street Church; Edward Everett Hale, the author (The Man Without a Country) and philanthropist, at the Charles Street Gate; and the abolitionist senator Charles Sumner and the Civil War hero Colonel Thomas Cass, along Boylston Street.
The park contains a special delight for the young at heart; follow the children quack-quacking along the pathway between the pond and the park entrance at Charles and Beacon streets to the Make Way for Ducklings bronzes sculpted by Nancy Schön, a tribute to the 1941 classic children's story by Robert McCloskey.