I found a nice collection of items when searching through the area where I am building my new raspberry garden. There is a 1936 US penny, an old button, a toy cannon that was built in the early 1940's a reed from an old reed organ, an old hinge, a marble, some nails and a few other things. The marble is not metal, but I found it while metal detecting so it still counts.
Update: from another website, I see that the button was likely from the 1830's. That would be the oldest thing I have found so far.