Still in development but very interesting to consider, by @delcano@mastorol.es

I've written an ActivityPub server which only allows you to post messages to your followers. That's all it does. It won't record favourites or reposts. There's no support for following other accounts or receiving replies. It cannot delete or update posts nor can it verify signatures. It doesn't have a database or any storage beyond flat files.
But it will happily send messages and allow itself to be followed. This shows that it is totally possible to broadcast fully-featured ActivityPub messages to the Fediverse with minimal coding skills and modest resources.