I was out hiking along a 400-foot cliff, when I found this GREAT section of climbing rope securely tied around a stout tree. Other end was going down over the cliff, but for some reason I couldn't pull it up. So I just cut out the section between the tree and the cliff. Got a good 15-foot section of slightly-used climbing rope. Oddly, there was this weird screaming sound that came from down the cliff, followed by a loud "SPLAT". Never could figure it out.
Okay--that didn't really happen

--but I figured a sick joke might get a laugh or two. Kidding aside, I did once find a Scout-type pocketknife, with decent Sheffield carbon-steel blades, on a trail in northern Arizona some decades back.
The best "lost and found" story I have, though, is my dad's. Wasn't on a trail, but in his carport. One night he heard a noise in the carport, and went to investigate. A couple of teenagers were in his carport, apparently trying to get into the cars he had parked there. When he surprised them, they took off. One of them had apparently come on a mountain bike, and abandoned it when he took off running. My dad took the bike inside, and later turned it over to the police. The owner hadn't bothered to use the local "mark your bike for security purposes" systems, and nobody claimed it at the police station for the required number of days--so my dad went in, at the end of the claim period, and the cops gave him the bike back. We now have a mountain bike, and an even better story--I can tell people about the time the burglars visited our house and left us a mountain bike.
Thanks for your generosity, by the way! And convey our thanks to your 5-year-old, too. Make sure you teach him plenty of these survival skills. First of all, he'll LOVE it, and it'll be great dad/kid time that he'll remember forever. Second, it'll teach him life skills that a lot of us are sorely lacking. I do this with all my kids, and there is hardly any activity that they prefer--kind of blows me away how much they eat that stuff up. (Sure you don't want to keep the fire piston for him?)