@evan one of the things I loved about early Twitter was the connected things - plants, ferries, etc. @andysc had a concept of "blogjects" -> "tweetjects" back then. ActivityPub absolutely can be another network where these things could be useful.
From a pure AP standpoint, it would be interesting to think about how to implement (say) ActivityPub C2S from something like Arduino or #micropython on a small device, or indeed from @nodered as a concentrator of sensor data.