These were found in a park in Pittsburgh, PA. From a distance they looked almost like plums, but they're actually hollow, paper-like covers that contain a hard pea-sized seed.
Most likely a member of the genus Staphylea. Since you are in Pittsburgh, PA, it's most likely Staphylea trifolia L., or an introduced species.
By the looks Of it,it mostly is a hop hornbeam Which are indeed found in pittsburgh
But i Still cant confirm it because they are no photos of their leaves and any twigs or the whole tree, this info would be great to identify it and do you mind mentioning the name of the park you've been to
Can you confirm whether the tree looked like this