A small canteen-style place on Lækjargata, Icelandic Street Food does the simple things very well: lamb stew (kjötsúpa) in a bread bowl, langoustine soup, fish-and-chips, all for less than half the price of the sit-down restaurants around the corner. Free refills on the soup.
It is touristy but cheerful and feeds you without ceremony. A useful first or last lunch in town. Open late.