I really thought they were seed pods. And I didn't think they could be the result of a wasp, as Susan suggested in the comments, but guess what? They are!
This is called Oak Apple Gall and is an abnormal growth that happens when a wasp lays its eggs on the tree. I saw three on the top of a little oak sappling on a training run. Between seeing that they were from an oak and the information Susan gave me, I was able to find out the full story. Thanks, Susan!
I'd like to take Zack to show him the little tree, but we tried to get out on a run together, me with the jogging stroller and him on his bike, and it didn't work at all. Or rather, it worked fine if I wanted to walk briskly for .7 miles and then poop out (not exactly my plan). Maybe a family walk with a stop-off for swinging on the swings halfway there would be better, eh?