My first and most simple idea:
By attaching the link to the original full post so that people do not loose their content.
Even if it shows me a json telling me that my best friend was gifted a son, I would be very happy to know, no matter if styled or json.
Better would either be simply concat (the multiple values problem) and bridging to supported types.
Better would also be an additional generic inbox UI (like DM here) showing it as a minimal representation.
@sl007 @humanetech You keep saying that as if it's possible to have someone send you an activity your software doesn't support. The whole point of having capability negotiation is to solve this very problem before it has a chance to happen.