Not sure if a magnet is the way to go.
All the rage around here is vehicle wraps.
Many companies have their vehicles totally covered with sheets of vinyl with designs that are only limited your imagination. Even windows can be completely covered with the stuff.

Google "vehicle wraps" and see the possibilities.
Then look for a local vinyl sign shop that does this and talk to them.
You could work out a great deal with them, maybe even a free wrap with a free ad for their services as part of the total design.


Another way to go is a bilboard hauler.
Has a cab and a flatbed with a verticle bilboard that displays the ads.
They usually go up and down the streets during events that draw large numbers of people.


Like Ankesh suggests, you could charge more for these as well.