At some larger parks you may find stairs, these are great for adding depth into your workout and challenging your body in different ways! You could run sprints up the stairs, bear crawl down them, and get in a couple sets up push ups at the bottom. If push ups are a problem you could always use the stairs as a tool to get stronger. Put your hands on a step that allows you to do 8-10 push ups with correct form,(click this link for a video on push up form) then when that becomes too easy just go down one step and so on! Soon enough you'll be doing push ups on the the ground!
So there you have it, with a little creativity and some searching around your local parks you can build a gym out of anything at all! For more great ideas on training both indoors and outside take a look at the new Jungle Fit YouTube channel at www.youtube.com/user/JungleFit