Automation can help with routine things, and in areas where you have high density you can effectively have spies create missions for other spies: one person's mission is to take a picture and deliver it with a note for the next person, the note tells them their next mission, and so on.
For added fun, check things like Twilio and their telephony APIs. Phone calls and text messages can add a layer of realism.
The holy grail, as seen in Halting State, is to procedurally generate all the missions, so you get something like:
SMS text to player 1:
Go to place X at midday, write code 12345 on the ground in chalk.
SMS text to player 2:
Go to place X at midday + 10 minutes, reply by SMS with the code you find.
If you have sufficient density of spies, arranging that kind of interaction should be fairly easy. On the other hand, everyone that doesn't live in a large city with an active population in the game is going to have a more abstract experience - no finding secret messages other people have left!
One way to get around that is to find live online webcams - traffic cams, for instance, with a good enough resolution - and get the spies to leave something visible in front of them. Player 2 then just has to locate the area on the web, and view it through the cam.
For added fun, check things like Twilio and their telephony APIs. Phone calls and text messages can add a layer of realism.