Make a dam
Kids love stones and water, so this adventure will keep them happy for ages. If there's a group of you, it's a great activity to do together as a team. Find a small stream and lay some rocks across it in a line (we called them squirrel stepping stones). Then fill the gaps with smaller stones from the riverbed and watch the water build up behind your dam.
The water will be pretty cold at this time of year, so dress as if you're expecting heavy rain and have some nice warm mittsat the ready when you've finished.