Centered around a flea market, Monastiraki Square is busy with musicians, beggars, street performers, and tourists. Sometimes you'll hear fireworks going off here, for no particular reason. Sometimes there'll be full-on drum circles. The flea market itself is diverse. You'll find all the regular 'ol touristy souvenirs, but also many locally produced foods. Go to the cheese shop for cheese, the bakery for bread, etc. The meat market and fish market are also nearby, but those are only for the brave.