Near Syntagma Square, there are four Oriental restaurants, standing almost side by side. If you get tried of Greek food, svoulaki after svoulaki, you might find them attractive.
First, Japanese restaurant Furin Kazan. The cooks and some of the service staff look Japanese. The customers are not only Japanese, but also many Greeks as well.
The price is on expensive side - as you expect from Sushi restaurants -, but has a good reputation. (Reviewed here).
Apollonos 2, Athens
(Blue point in the map below)
This is Japanese and Korean restaurant Dosirak. It serves Korean food as well as Japanese dishes like sushi, sashimi, tempura.
Voulis 33, Athens
(Green point in the map below)
Right next to Furin Kazan, Chopsticks is probably the newest of the three and serves Chinese, Thai, and Indian cuisine.
Looking from the outside, the staff look Chinese.
(Pink point in the map below)
Recently a Greek (I think it is Greek, but not sure) chain restaurant + take-away place Noodle Bar opened at Apollonos 11 (the yellow marker in the map below). The food is vaguely Chinese and South-East Asian. The budget probably is between € 10 and 15 (check out the price list in their website).