I don't think there is risk of catching anything from the park other than things like contact dermatitis like poison ivy. But . . . make it special. If you are so young you have to sneak out and do it in public, maybe it's not the best time in your life to take this step. good luck
I want to start off by saying the safest place is in a comfotable place at home. I have had 'it' in many places. As long as you dont rub your parts on things or get caught you will be just fine. Safest way is a way that doesnt hurt either partner. Sex is meant to be fun....experiment...have fun...but stay safe!