Oh I love this! I've been trying to understand ActivityPub from first principles, looking for a really simple hackable server. (Really what I want is a server that lets me write the ActivityPub equivalent of CGI scripts to easily host diverse content.)

I've been failing to find a simple example server other than maybe Darius Kazemi's https://github.com/dariusk/express-activitypub

This little static file experiment is a great teaching tool. To folks complaining about conflating ActivityPub with Mastodon, part of his goal here is to do the minimum ActivityPub implementation so that his account looks a certain way on Mastodon. Other ActivityPub instances might be asking for different requests; he's doing the Mastodon minimum.