No this restaurant doesn't serve insects (although they are high in protean) and many Asian people swear by them! Its merely named after an Aesop's fable the Cricket and the Ant - I made that up, and have no absolutely idea why its named that other than trying to be cute and that's what this place is cute!
They also serve healthy traditional Greek food and have large salads that will run you around 8 euros but are a healthy alternative to some other things you may be exposed to. My girlfriend, Ads's sister, Mitsi, took us there and it was crowded and they even asked us if we had reservations which I thought then was a bit pretentious and still do. Still its healthy and you may be able to find a table so check it out!
There are two more locations as well: Ano Patisia: Papadiamanth Sq. 4 and in Halandri Agiha Georgiou and Aisxylloy 26.
Katselis Bakery: ( left) is a bakery which sells bread and coulouria too but also sandwiches and soft drinks. Katselis is about average for this type of chain type bakery place. Want to try the quintessential Greek item? Then order a cheese pie from a bakery - they make them fresh - some smaller places outsource! Goodies,across Ermou street, has an equally good selection perhaps.
Far right is Pita Pan, which is a play on words, to wrap up what is in reality, a souvlaki shop although this one, also a chain, which sells to a primarily Greek audience, and has three veggie options too and several types of salads. There is no where to sit however is any of these three stores but many office workers get food from here to go. You could easily eat in the center of the square in the park on a bench if you wanted to dine outside.
Incidentally, not far away at all, heading towards the Plaka and Monastiraki, via Nikis St., over on Adrianou St. (Hadrian's St.), there are even a couple of Asian restaurants too, a sushi bar and a Chinese to go style noodles joint. They are a couple of decent hotels over here too that you should consider staying at like the Hotel Plaka and the Adrian. Remember Harry's Greece Travel Guide will meet or beat any legitimate hotel price you show us.
Street vendors of course come and go. Here are a few that I saw when I was last in the Square. Lets start with my favorite the nut vendor. Nuts are a product of Greece so they are fresh and pretty cheap too! Try some! Then there is the nut vendor with popcorn and with chestnuts another local tree fruit.