Well, it's time to close the poll. I have to say that I got many more guesses than I expected and they spanned a much wider...variety than I expected. I'll go through the garage and see if I can find some piece of house history to send on to the winner (because there was one). Something not too valuable or too crummy, but something worthwhile. No guarantees though.
As I said, we found out what it was because Ms. Huis talked to the neighbors about it. Now, it didn't come up in conversation because my wife said, "My husband has this crazy fixation with a stupid picture. What's this round thing?" The conversation came up because our oldest daughter was playing on the round thing that got moved to the neighbors' yard.
Yup, it was a trampoline. When the renters lived here, they had it, and when they decided to move on, they didn't want to move the trampoline, so they sold it to the neighbors.
Congratulations to Pusher for the correct guess. Also, second place clearly goes to NoNick. And I have to say that I thoroughly enjoyed the guesses.
Hope everyone has a warm, sunny, fun and safe Memorial Day weekend (if you live in the U.S.).