Athens Metro is cheap and efficient.In addition,

you can access many beautiful islands from Piraeus. One thing I enjoyed most in Greece was to dive and catch sea urchins. My feet are still hurt now by their stings  LOL.

It's a beaustiful country with very friendly people, wonderful food, morning coffee, stunnig scenery, and so on - too much to enjoy. I'm sure you'll have a nice trip.

P.S. English is very popular there for this is a country liberated by Britain and Russia. Lord Byron, a British poet, died there for fighting against Ottoman Empire in the Greek War of Independence.

Athens Metro Map