On a budget, Charles River Esplanade still earns its price.
Free
The city's best free outdoor hangout — joggers, picnickers, sailboats, and the classic Back Bay skyline-across-the-water view, capped by free July 4th and summer concerts at the Hatch Shell. The catch: it's a linear park with no single 'sight,' it's exposed and cold/windy in the off-season, and it's a locals' commute-and-exercise space more than a tourist stop.