If you are looking for things to do at night in Athens, head over to the Mnisikleous Street stairs. Restaurants line this set of stairs on the top end of Mnisikleous Street, drawing a crowd in the evenings.
The restaurants vary in quality, although some are quite good, but the atmosphere here is hard to beat. It's extremely casual and relaxed.
In fact, when the tables are full, people take to sitting on cushions on the stone stairs, and small knee-high wooden benches are brought out and placed on the stairs, creating an impromptu table. People are often tightly packed in, making for an intimate and friendly experience.