Athens was not my favorite place. We accidentally stayed 2 days here before flying out to Santorini and I promise it isn't worth the stopover (unless you've got LOTS of time).
The highlights for us were the Acropolis (which had a pricey entrance fee) and watching sunset from Foligpalo Hill. Both these things can be done in a few hours and after that we literally
couldn't find anything to do.
The city has a great metro system which makes getting to and from the airport really easy. We stayed in the San Remo hostel which was a bit of a shithole but it was clean and had nice
owners so we were happy there.