A few minutes from Syntagma Square, down Metropolis Street, you can find the city's cathedral, a huge, not particular attractive, 19th-century building.
But much more interesting is the adjacent "Little Cathedral" of Panagia Gorgoepikoos, a very pretty, little, beige stone, 12th-century church. And I do mean little. Not only is the church dwarfed by the adjacent cathedral, but like some other early Byzantine churches it is constructed on a very small scale almost like a church for children. The exterior has some nice carved friezes.