Satnav Spy Game.
The main thing needed for this game is some manner of satnav/GPS tracker thingy that can be installed in three cars, allowing the drivers to see not only their own position, but also the positions of the other two. I don't know if such a system exists, but if not then some boffin can probably put it together.
The game goes as follows: take the three satnav tracker equipped cars, plonk them down in three separate locations in the outskirts of a city. Provide the drivers with digital cameras, and here's the challenge: using the satnav they're supposed to navigate through the city, and take photos of a number of predetermined things - buildings and whatnot. Each (clearly) photographed target gives, say, 5 points.
Meanwhile, there is another challenging thing: if a driver manages to take photos of the others, he will get 10 points per picture (provided it's clear enough for the driver and/or his car to be recognisable). This should be both easy and hard - since they can see each other's positions they can stalk, but the stalked can see the stalker coming, and either flee or make their own pounce.
The game ends after a specified period of time, and the one who's gathered the most points wins.
Now, about the cars. Since it's a spy game, it could be a way to determine the best Bond car, or the best spy car in general. Or it could be a way to determine the most discreet car - or the most discreet supercar (it would be quite amusing seeing, for example, a Pagani, a Koenigsegg, and a Ferrari Enzo trying to lurk unnoticed). Or it could be regular cars with numbers/text on them, or obnoxiously bright colours, so they're easily recognisable. Whichever works best.
I haven't actually played this game myself (I don't even have a car), so I don't know how well it'd work in practicality, but I'd love to give it a go. It might need some tweaking, and I somehow I don't think playing it during the rush hour would be such a great idea.
Also, there is the location to consider. A city none of the drivers is too familiar with could even out some odds, and personally I'd like to see it take place in Stockholm. Not too small, not too big, with a bit of geography adding some spice to the game, what with the city being built on islands, interlaced with high bridges, and deep tunnels where the GPS signal might be lost.