Brain-storming

Here's a remotely viable idea:
Urban noughts and crosses/tic-tac-toe

Utilising Melbourne's Hoddle grid system, the CBD is divided into a "board" that is viewable on the players' iPhone. The players, wearing tracking devices that can be received and recorded on both screens, move along the grid, either circling a block or making a cross by foot. This may involve going through shops or down laneways to complete the shape. If the shape is too vague, the application will disregard their efforts.
Say if the game were timed, and the player did not complete a recognisable nought or cross in the square, or if the shape is too small, the turn would go to the other player. Players must move along the grid lines themselves to move from one square to another, where their movement will not be recorded as a "go", or even detected, as this would allow to their opponent to see where they are planning their next move. After a nought or cross has been made, the next player is green-lit to record their movements in the square.
Options for this include the size of the "board", and with it, the time constraints. Because Melbourne has a tram system, the lowest level would be played in the CBD along tram-serviced streets, with more complicated games taking place in areas without trams. On harder levels, the time constraints would require the players to run.

As we know, first ideas are never the best, so let's hear some more.