ActivityPub Academy is a customised Mastodon instance designed to let you explore the ActivityPub protocol interactively.
The protocol is brought to life by showing Activities sent between different instances in real time! Sign up for a fully functioning Mastodon account (accounts are deleted after one day, but you can always create a new one). Follow other accounts, create posts, boost & like, and see the effects on the protocol visualized.
For more background, read the introductory blog post.