I think pullups are a great workout at a park. A lot of parks have some form of them. I like them becaue they engage your lats, mid-back, rear delts, biceps, forearms and core. You can couple these with push-up in the park for a very complete upper body workout outdoors. With push-ups you hit chest, shoulders, triceps and core so I often like to superset the two together. Add in a few other random exercises depending on whats available at the park and you really can have a great workout.