When you walk across the archaeological sites at the historical center of Athens you see many trees and bushes. But in general, Athens is mostly nude of green. So, just enjoy the view and the sense of the landscape as it was during the ancient times going to Monastiraki, Plaka, or to Filopappou and to the Acropolis.
You can rest many times sitting on a stone, a bench, on some steps and then keep walking until you feel tired and stop again. If you want to have a coffee or a drink, avoid the cafeterias close to the Acropolis, they are expensive (of course there are some that are expensive but, at the same time, splendid as Dionysos- ask for its location.
You should better choose some other bar, pub, coffee shop or ice- cream shop in the neighborhood but a bit away. Walk until you like some of them and then have a look at the prices and decide. And yes, you can smoke at every shop while sitting outside.