That's the one I was thinking... there are many Phylloxera species that make simple round blobs on the upper side of hickory leaves, but P. caryaeglobuli is the biggest, at 1/3-1/2". This one looks like it probably is that big, and it looks just like the ones on that page, assuming they're correctly identified. Unfortunately, I wouldn't put 100% faith in that assumption, because the image just to the left of that one is misidentified. Phylloxera caryaefallax makes a gall that is "conical above and much more prominent than the convex undersurface..." (Felt 1940) while the ones pictured are detachable galls, entirely on the underside: the work of a midge, Caryomyia caryaecola.

I would be sold on this one being a Phylloxera gall if we could see the underside, which would have a slit at the bottom to let the phylloxerans out:

(Incidentally, it looks like I may have misidentified that one, because John estimated the size at only 5-7 mm. I'll have to re-think that. Same goes for the linked image of mine with the gall torn open. Definitely a Phylloxera though.)

I'll check again this summer and get some photos of the underside if there are more of these on the trees. I think there's at least one hickory tree left in the park. Last fall the village decided to mow just about everything to the ground in this park to get rid of some invasive species...

Thanks for the link. I've seen quite a few of these galls and now maybe I can identify some of the ones I've seen. Here's the link for those who prefer to just click rather than paste and click:)

Plant Galls