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 midda

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!