@Lizzie an idea suggestion around the Airbnb events for some cities. In Porto for example, in the peak of the summer there's thousands of tourists hosted in the city through Airbnb. Why not get them together around the events? Specially at night and around the table is a good way to some hosts meet together. This could even work better with business travelers as they often travel alone.
Fifa Euro 2016 is coming and i do know for experience that foreigners in the city do like to know what places (restaurants, bars, coffees) are the best to see this matchs. Porto receives hundreds of French/Spanish tourists that love soccer. This could be easy to set up and celebrate the event as embraces hosts from Airbnb for a different experience awhile travelling.
This event is just an example but big sport matchs are happening all the year long.