Along the leafy banks of the Charles, the Esplanade Association’s River Fit series offers free outdoor classes from forest-bathing nature therapy to cardio dance, yoga and HIIT, set near the iconic Hatch Shell. Sailboats tack across the river and joggers stream past as instructors guide you through the evening session. Presented with Blue Cross Blue Shield of Massachusetts, it is one of the prettiest places in the city to break a sweat.