I highly recommend you stop in a Gyro shop for some of the best, most delectable, junk food gyros ever.
In case you don't know what one is, or you don't know how they make 'em in Athens, here's the drippy, delicious deal: a grilled pita rolled around thin, mouthwatering slices of grilled meat cut directly from an upright rotisserie, fresh lettuce, sliced tomatoes, onion, and cucumber, hot french fries (yes, on the gyro), tzatziki, and in my case, hot sauce. (Most places serve the traditional lamb, but others have beef, pork, or chicken.)
With a beer or a coke, it's about the best greasy, easy meal-to-go you'll find in town. Avoid American fast food, Athenian gyros in are just so great!
I ate at least one daily throughout Greece, and my record for the most consumed in a day topped out at 5 (in Matala, Crete). That many I do not recommend!