I folded over the edges of a piece of duct tape, leaving a sticky strip in the middle, then as we walked the kids gathered interesting leaves, twigs, bugs (!), or whatever else struck their fancy, and stuck it on the tape. When we got home, we put clear plastic wrap over their finds, then bordered the bracelet with another strip of duct tape on each side. Punch a hole in both ends, tie with yarn and -- voilà -- a beautiful nature bracelet.
Kate was quite happy that I happened to have purple duct tape on hand.
It was a fun project and really kept them engaged on the walk.
Of course, I can't take credit for the idea; I read about it somewhere. Instructions can be found HERE.