To make it more challenging, I'd suggest banning the use of GPS, since they are usually programmed for the shortest distance between given points. For that matter, ban laptops and smart phones as well to eliminate the use of mapquest or other on-line mapping services. Make the navigators WORK for their accolades.
I've got that figured out as well. At 2 of the 5 check points, the drivers will be given a "bonus" instruction. They won't know where the bonus instruction will take them until they get it. If it's a place they haven't been to yet, then they're in luck. If they've chosen to visit a checkpoint in that direction already, then they'll have to do a bit of back tracking. It all comes down to the luck of the draw.
Another thought I had was putting people at the check points and handing out direction to the next checkpoint. But not all the directions will be the same. You'll still visit all the same spots, but the luck of the draw comes into play.
And besides, if someone really wants to use a GPS/laptop/smartphone to win a silly rally with no real prize, then let them. If they need that much of an ego boost, maybe they shouldn't be driving a MINI (or Mini) in the first place!