We visited a local national park recently and I was looking at a display of Junior National Park Ranger badges and thought it was an interesting program.
Our local park has such a program and from what the Ranger there told me and showed me, they give you a short activity booklet that your child works on [ages 5 to 7]. There is a different activity book for older kids. He said it need not be done in one visit, that it could be over the course of a few visits. After the child completes it, they get a pin [I'm not thrilled about the rewards aspect of it, but I suppose if the child is motivated to do it for its own sake and not just to get the pin, that I can deal].
The link above has a list of all the National Parks that have this program and links to them.