Do a food tour! I say that about everywhere though, great way to find some hidden gems and stuff your face.
Otherwise the usual tourist stuff - Acropolis, Panathenaic Stadium etc. Temple of Olympian Zeus was covered in scaffolding when we were there, and somewhat underwhelming, but you can see it from outside the fence without paying anything. I also enjoyed the botanical gardens but that's also something I try to visit everywhere I go, love a garden