I'm really looking forward to doing nature walks with my kids someday. I've been collecting nature identification books (birds, rocks, trees, flowers, etc) for quite some time in anticipation of when we will be able to use them as a family.
At 18 months, however, M. is not very up for that kind of thing. The other day, though, we did go on a simplified nature walk. I labeled two bags as "sticks" and "rocks" (the two things she interacts with the most when we go on our walks around the ranch. 
I was pleased that she seemed to understand the concept and after a little explaining and demonstrating, was taking initiative to put the correct objects into each bag. At first she seemed to be having fun with it, but with many sorting activities we've done together so far, it got old after about 7 minutes. Oh well, it was fun while it lasted, and we enjoyed the rest of our walk anyway.
I was pleased that she seemed to understand the concept and after a little explaining and demonstrating, was taking initiative to put the correct objects into each bag. At first she seemed to be having fun with it, but with many sorting activities we've done together so far, it got old after about 7 minutes. Oh well, it was fun while it lasted, and we enjoyed the rest of our walk anyway.
(Some sticks were a little too big for the bag)