I visited the woods with my nieces a few weeks ago. They enjoyed the nature pretty much of course and were happily hiking until we decided to take a break. We (the adults) began to fall in this comatose state of drinking, eating and talking :D However, the break took a little too long for our youngsters and so they began to get bored. 

Are there any games I could have given them which can be played with what the nature provides (common hiking equipment etc. is ok, too :) ), doesn't leave a trace (or at least not _that_ much) and is safe for the kids (in terms of "lost on their own in the wild" etc. - negative example: hide and seek)?