I have 2 camelbaks and love them. We will definalty be taking one of them to the park. One is holds 2 liters of water and can carry a small amount of stuff.. I would say a light jacket and a couple of paper backs maybe (not that you would need the books but thats just the size), the other hold 3liters and is the size of a regular school back pack. You can get ones that also just have the bladder and a small side pocket for keys or an ID.
I would not get one of the ones at
walmart, before getting the second for DH I looked for a cheap one at walmart. Was not sure if my dear city boy would like hiking. They were so cheaply made I just couldn't do it. I ended up springing for a 3liter camelbak one. Figuring if he didn't get the hiking bug I could always use another pack. This being said they do run $60 too $100+. So if your not going to get much use out of it I would try the ones a at Target. Someone on another thread said they were cheaper than the actual camelbak brand but of a much better quality than the ones found in walmart. I found the best deal for my second from
http://www.backcountry.com/ I dont get kickbacks from them, just a good company.