Last week I went to an Asian Feelgoodfood-restaurant-more sort of-Imbiss called Coa. The atmosphere is very nice, stylish and simple at the same time. You can sit outside, and the prices are really moderate (given that it's in the Schiller passage right by the Börse). I can recommend their homemade Ice Tea. Takeaway is available and nothing costs more than €10. They are quite accommodating if you want to request a change - I had tofu with my dish for instance - and the food is presented very nicely.

Schillerstraße 4
60313 Frankfurt

Mon – Sat: 11:00 – 23:00
Sun: Closed

Directions:

From Hauptwache station, turn up Schillerstr. Coa is on the right-hand-side just before you reach the Börse.

Nearest S-Bahn(S):

Hauptwache (S1-S6,S8-S9)

Nearest U-Bahn(S):

Hauptwache (U1,U2,U3,U6,U7)
Eschenheimer Tor (U1,U2,U3)

Nearby Attractions:

'Börse' (the Frankfurt Stock Exchange)
Escherheimer Turm