I saw these guys hanging from a tree along the side road one day and thought, "Oh, wow! Wild grapes!" When I finally stopped to look at them close up, I realize they are plums, wild or otherwise.
We had plum trees on the farm where I grew up in Wisconsin. I ate and gathered too many as a child and now can't face them. Guess I blocked them from my memory. But to my credit, these are the same size as grapes.