There probably aren't that many of you, who both remember Dopplr as well as know about ActivityPub, but: could this be a perfect fit? I was thinking about implementations for a decentralized nomad/travel app for quite a while (i.e. see where in the world your friends are and get notified of correlations in your travel plans). Was going to use public RS profile documents, but AP would likely scale better.
Privacy would yet again be a concern, because the best feature on Dopplr was a config for only showing months (as opposed to specific dates) to people you aren't friends with.
One example for how it worked is the widget you could add to your website. Here's what it looked like on my personal site: