If you can rely on the participants having cameraphones (or cameras!) then a Lucky Dip one might work.
Choose some locations - these could be specific (St Anne's Church, the Dog and Partridge, Toad Hall), generic (a phone box, a farm animal, a bus) or more offbeat (something triangular, something gold, something rare) depending on your local area.
Type up the locations on cards (an easy way is to print a grid on A4 paper and cut them up) then sort them into envelopes - maybe 10 per envelope.
Each team gets an envelope at the start and has to find and photograph as many locations as possible in the time allowed. The generic ones add an element of unpredictability so it doesn't just become an orienteering exercise. Ideally you want it to be very hard to get the full set so that all teams don't get 10 points.