So far, I've tried and enjoyed:

• Mastodon, a twitter-like microblogging platform
• Peertube, a peer-to-peer youtube-like video posting and streaming platform
• Pixelfed, an instagram-like image sharing platform
• Matrix, a federated IRC-style chatroom protocol (not really fediverse/activitypub, but still interesting)
• Wafrn, a tumblr-like with support for Bluesky's AT protocol too!
• Misskey, a unqiue social platform with lots of features, very popular with the Japanese art crowd
• Mbin, another reddit-like with lots of interesting features and a nice interface.

And there are still plenty more that I haven't really tried yet! The fediverse is pretty huge!

As for recommending specific sites, that's a bit harder since it's healthy that not everybody is on the same server. So I would just recommend picking a reasonably popular server with open registration.