I'm happy to announce that my barebones ActivityPub server, implemented in Node.js/Express, is now open source! I intend for it to be a reference implementation for developers who are adding ActivityPub to their own services, but also it is an application server that you can build on if you want.
In other words, t's a few hundred lines of code that lets you create Mastodon-compatible accounts that can accept follow requests and post stuff to their followers.
I actually built a little toy service (that I will also eventually open source) which converts any RSS feed to an ActivityPub actor that you can subscribe to in Mastodon (or any other AP-compatible client).
Play with it if you like! It is SUPER rough and most feeds end up horribly rendered in Mastodon but it's still kinda cool to see it work: https://bots.tinysubversions.com/convert/
This is my personal Mastodon.