I inherited an easy summer program from my predecessor, who had planned a "Nature Walk Scavenger Hunt" to be on the schedule during my first week here at my new library. There weren't many details left behind, but this is what we ended up doing.
First, we met in the library's programming room where I gave everyone a scavenger hunt handout with a list (including pictures for pre-readers) of items that we were looking for. I also gave everyone a paper lunch bag to use for collecting things we could bring with us, and we discussed what types of things are okay to pick up (sticks, leaves, rocks) and what types of things are not (flowers from someone's garden, live animals). Then we started our walk.
There were a lot more people on this walk than I had expected, so I sent an assistant to lead the walk three blocks to the park while I played crossing guard at each intersection. Once we were at the park, we talked about what we found, then turned around and walked back, hoping to find those last few things on our list.
At the library, we used our nature items to make paintings on construction paper. We did this outside, both to prevent the mess inside and also because we had too many people to comfortably fit in our programming space. The kids made pictures, I made appreciative comments, and then most people left their pictures to dry as they went inside to check out books.
All in all, I would say that this was a very successful program and well worth doing, especially if your library is in an area where it is safe to take school-aged kids on a brief walk around town.