For the Norwich AlleyCat Bicycle treasure hunts that I organise, I have started to use Silva oreinteering control card punches as proof of passage at some of the way points. We have found them to be particularily useful for the out run and the home run sections where the cyclists want a couple of opportuinties to pedal fast and furiously without the need to stop and take time solving a clue, and those who did the December Alleycat liked using the punches on these sections.
These orienteering punches have the advantage that I can discretely locate them out at a point on a cycle track or other location where it would be difficult to make up a clue for. Because each punch has a different pattern of pins it virtually impossible to cheat. At £32.24 and free P&P for a pack of 10 they are inexpensive.
http://www.aboveandbeyond.co.uk/.Silva-Control-Punches-B-Box-of-10-_7318860117183.htm
The Silva punches come with a set of holes in them for either attaching them to a length of cord; paracord is ideal, or they can be screwed to an object such as a wooden fence post etc.
The January Norwich AlleyCat, will be a bit of an experiment, as rather than a mixture of 20-25 clues and photos, it will be composed of just 7 clues to solve at various locations, and 7 punches to mark the task sheet at other locations, and only one photo this time. If this formula proves sucessful then I will continue to use it, and hopefully either the April or May Alleycat will be just a navigational challenge only using the clues to direct the entrants to the control point punches so they can mark their cards as proof of passage.