Put your money in the machine which is set up to allow you to punch in your route and dispense a date/time stamped ticket for your ride that day.
For commuters, a card or some kind of RFID device (I do know that MARTA has been around for a while, but hey how about updating the system every now and then?) to validate their route and spit out a ticket for that day's ride.
I forgot about the need for the city dwellers to have a flexible system to ride at random, but current technology could solve that one very easily, I'm sure.