There are plenty of valuable items that people drop every day. A simple metal detector is all you need to find them and make some extra money.
Don't have a metal detector? Don't need one. Just grab the Metal Detector app.
No, I'm not joking. Newer Android smartphones contain a compass for navigation. It's not difficult to tweak it to detect nearby metal objects.
You will need to find a comfortable way to hold your phone near the ground, however. Walking around bent over just looks odd.