I've got this retirement plan - travel to the world's beaches with a metal detector and see what I can find. Ever since I was watching Pawn Stars where the guy brought in a gold bar from a shipwreck, I've had this small dream.
I found a treasure hunting forum, treasurenet.com, and was really amazed at the stuff people find - everything from civil war buttons, bullets and buckles, to old coins. I realize that I would be far more likely to find a bunch of aluminum bottle caps than gold bars, but it still seems like a nice mellow hobby, especially if you had a little bit of land out in the country.