As a means to motivate myself to work on it, here's my attempt at an Activity Pub server: https://a-puc.social/. There's a write up on the page about it, but the gist is that I wanted a front end that didn't bring my browser to a crawl and wasn't designed around the contemporary idea of "only the present matters." It's still very early on, so don't expect to be using it any time soon.

Also see @ne's active project: http://wormhole.nekobit.net/. We both had almost the same idea simultaneously, to make an Activity Pub server in C++. He'll probably finish his first though, so make sure to follow him.