I layer my clothing. I would wear a t-shirt under a sweatshirt and then a windbreaker on top of that and just take off layers when I got warm and tied them around my waist.
I usually bring a drawstring bag ( similar to this one:
http://www.modells.com/product/index...ntPage=family) with me to the parks filled with my wallet, camera, extra film, chapstick, sunblock, and a snack.