I don’t know of such an alternative. A quick solution would be to use something like GeoNotes to take geolocated notes.

As far as a self-hosted solution goes, I’d just like to point out that you wouldn’t need a self-hosted database of places. You could query Ouverture (or Google, OSM, etc.) for places near you, and you’d just need to store the check-in on your server with a basic API. This is an interesting problem, and not super hard to implement.