Check out this invaluable resource fromt A.M.C. I hope it is helpful!
Another activity I have seen done with older kids and could see it being done with younger ones too, involves only a pencil. Bring an unsharpened pencil w / you. And as you stop to take breaks have your toddler turn around as you hide it. Pencils camoflage well in the outdoors. Depending on how old your toddler is, perhaps have he /she hide it at the next stop!
Any time you can make a game, sing a song etc, when hiking w/ little ones is a win in my book!