First We'll Map Manhattan!!
Then We'll Map Berlin (Toronto)...to paraphase Leonard Cohen Song
We Make Money not Art (I think I might have to change my name to "I make Nothing, and even less Money"...)has a story : A Street History in Semacode:
a digital graffiti project installed along the route of the former 3rd Avenue elevated train line throughout Lower Manhattan. This train line was part of a religious boundary called an eruv for the immigrant Polish Chasidic Jewish community in the first half of the 20th century.
SemaCodes being the same thing as QR Codes
Then I remembered seeing this story : We'll Map Manhattan in the NYT's, about a suggestion to to make a literary map of Manhattan -- not of its authors' haunts but those of their characters.
Ok, what about combining the 2 approaches. Take your City - say London - with SemaCode or QR Code stickes which takes you to the entry about Sherlock Holmes did X (in Novel Y) here, and also in the "Real World" this historical event happened across the road. All allowing the Literary, and historical, to overlay on the day to day world.
This could also be combined with - or replace to some degree - the PDA Tour guide