I did something similar - on Wednesdays we would give seniors a
token for a free wash if they bought the top package ($10). It ended up as the equivelant of 1/2 price, but only if they selected the top package with their
token - not all of them did! It also brought them back to the wash if they had two cars and they would go for the
vending, vacs etc. with another vehicle.
I am no longer fully attended, so I stopped this promotion, but I do offer seniors a 15% discount on all
token purchases all the time. Basically, they get my top wash for $8.50 any time they want it. They seem to like this since it is not restricted to a certain day.
Good luck with your idea.