Thanks John I was going to suggest Aris. There is also layar https://www.layar.com which had been around a long time. I recall some people doing stuff with it at UBC.
Also maybe some stuff at NYU AR lab http://mobilearlab.bxmc.poly.edu
You might do some low tech things like putting QR codes in public places that could be connected to some kind of clue or scavenger hunt that points them to next location.
Or maybe with google street view, have them find a place that they see there, and reproduce the image with a photo of themselves in it (eg try to recreate the image)