Are you looking to do someting akin to sprinting - ie tabata intervals, fast movement, or are you looking to do something that counts as Lift Heavy Things?
I'd personally recommend some bodyweight excersises, some squats, push-ups, dips, pull-ups or variations there off, depending on what there is at the park to use. THe primal blueprint fitness book is a good starting place, 4 movements, fairly high reps, so it gives a combination of strenght and edurance, and free
There are also loads of free training apps for smart-phones and plenty that cost a few dollars, I've got a circut training one that gives various levels of work-outs either Tabata intervals, or slightly longer HIIT intervals or an AMRAP timer (as many reps as possible)
You could do a combination, a round of body weight excersies, a round of tabata intervals, and repeat if you want/can
You know all those pictures of Adam and Eve where they have belly button? Think about it..................... take as long as you need........................