Well, we did it! We had planned an outdoor birthday party at the park, but the day before, it looked like it was gonna rain the day of the party.
Then, the day of the party, it rained. So, we had the party inside. We moved around furniture, shoved a bunch of junk in a room we closed off, and cleaned some surfaces that we never clean. (I gotta say, though, having to vacuum every day definitely made this lift easier.) Katt came up with an activity station, and we made the guy's existing toys the "other stations."
We had 21 (!) people in our apartment. I think they were all happy? There was only one meltdown and no injuries. That seems amazing with 10 people under the age of 5 (and is to the credit of the various parents, rather than anything we did). They played with trains, made jewelry, played avant garde atonal music on the toy piano, fabricated food with Play-Doh, did some kind of glow stick hangout in the dark, and literally ran down the hall screaming, repeatedly.
They were happy, the guy was happy, and the parents seemed to have a pleasant time, even though I struggled with everyone's name. This is a miraculous outcome for decidedly non-party guys like Katt and I!