This study used observational methods to examine physical activity (PA) and selected correlates in 28 parks in Tampa, Florida, and Chicago, Illinois. We observed 9,454 park users within predetermined activity zones and coded their activity as sedentary, walking (i.e., moderate intensity), or vigorous PA. In Tampa, higher temperature, unorganized activity, lower amounts of shade, lower neighborhood income, Hispanic neighborhood ethnicity, male gender and child age group were significantly associated with walking. Vigorous activity was not associated with income and ethnicity. Morning hours, unorganized activity, lower neighborhood income and African American neighborhood ethnicity were associated with walking in Chicago. Vigorous activity was associated with children, lower neighborhood income and African American ethnicity. Findings from this study can inform policy decisions and future research directions.