An interesting one from the New York Post about a man who makes a living prospecting for gold and diamonds in Manhatten.
Link: Got his "mined" in the gutter
Of course, this isn't without precedent, there have long been panners for gold wherever there are big cities and clearly defined jewelry districts - here's an article on the gold panners of Mumbai:
Link: Panning for gold, Mumbai streetsweepers watch for all that glitters
And if those articles fail to inspire you, in Japan recently they've taken to mining the precious metals out of sewage,
Link: Sewage yields more gold than top mines
Which might make a fun afternoon for the kids, panning out the remnants of bran muffins and pancakes...
Of course, if you wanted to keep them interested it might not hurt to "salt the pot" a bit with this or some other edible accessories here or here.