These are one day scavenger hunts conducted in particular cities that offer fun and competition for photographers while providing photographs for Wikipedia articles that lack them.
The first such event was Wikipedia Takes Manhattan, which was coordinated by Columbia University and New York University students. The event had a target list of 188 locations, of which 92 were successfully captured and added to Wikipedia.
All Wikipedians and non-Wikipedians are invited to participate in team of up to three (no special knowledge is required at all, just a digital camera and a love of the city) [...] There will be prizes for the team who takes the most targets.
If you’d like to participate in one of these hunts, you could try to start one for whatever city you’re living in!