There's a couple different implementations that might be interesting to cover, depending on what you had in mind:
Friendica, Hubzilla, and Streams all support Activity Pub but focus more on a Facebook style social media instead of a standard microblogging platform.
Writefreely, Plume, Wordpress (via plugin), and Ghost (via plugin) all support activity pub and focus on long form content.
Peertube focuses on longform video, Loops on shortform video, and Funkwhale on audio based content.
There's also bridges from Activity Pub to AT (Bluesky) and Nostr as well.