What is your favorite exercise at the park?
While you have have received some great answers, I do most of those exercises at home. I reserve the park for Exerstride Nordic Walking. This is a method of nordic walking that uses Exerstride Fitness walking Poles. These poles have undergone over 30 independent studies, including one by ACE at the UW-LaCrosse human performance labs.
All of the studies show an increase calories burn of 20-50% with no increase in percieved exertion.