I’m old enough to remember the trick of checking the coin return slot when passing by pay phones – you can still do that for vending machines, but spare change isn’t worth what it used to be. And there’s far too much competition now for redeeming soda and beer cans/bottles (though if you live in a household of heavy drinkers of either sort, you can probably redeem enough for an occasional latte).
But if you want better odds of finding a minor windfall, do this:
Visit bar parking lots at dawn (or before, if it’s safe) on Sundays!
My father had a regular route in his Lynn neighborhood, and would frequentlyy come home with $10-$25 left behind by bar patrons (who probably shouldn’t have even been driving) missing their pockets or pulling out loose bills as they fumbled for their car keys.