We examined differences by sex and race/ethnicity in the observed moderate- to vigorous-intensity physical activity (MVPA) of youth and adults in diverse areas of 4 parks in Kansas City, Missouri, in 2009. Male youth were more active on playgrounds and pools or splashpads than female youth. White youth were less active than nonwhite youth in open spaces and on paved trails. Male adults were more active in open spaces than female adults, and white adults were more active on paved trails than nonwhite adults. Understanding variations in MVPA between user groups can inform park design efforts to foster increased activity among all visitors.