I hadn’t really looked, but it turns out that Mastodon has incorporated microformats. It has h-feed and h-card, h-entry (a status), and h-cite (a boost). Plaint text properties (p-), e-content, and link properties (u-) are implemented. Indeed, they all surface when looking at a profile’s HTML source. This is what makes it possible to e.g. follow Mastodon feeds as h-feed, next to the existing RSS output and ActivityPub, and that e.g. Brid.gy can do its work to carry over any interaction on a Mastodon post to a blogpost here.

What I haven’t found was what I was looking for.
The ActivityPub protocol in its specs has several so-called Activity Types that drew my attention:

In short ActivityPub supports FourSquare and Dopplr like check-ins and travel plans. I’ve recently added that to my site in terms of microformats and was still wondering how to create a useful stream for it. I’ve been thinking about an OPML outline with schema.org attributes, or a dedicated RSS feed or h-feed. An ActivityPub stream might be of interest too, or even more. There’s a PHP implementation of ActivityPub that includes these Activity Types as well, meaning there’s potential to experiment for me.

I wonder, are there any actual implementations of these ActivityPub types currently?