A playground can do more than make you feel like a kid again. Find one with a set of monkey bars and you've got everything you need for a quick, effective workout. Pull-ups are just one move you can do. See below for some more creative options. Repeat for reps or time.
Hold your body parallel to the ground with knees fully extended. Draw your knees to your chest, then extend again.
Spread-Eagle Leg Raise
Spread your legs to form an inverted V, then raise them until parallel to the ground. Slowly return to the start.
Lower yourself over the bars until your arms form a 90-degree angle, then press up until your elbows lock out.