Ricardo's place is situated about 20 minutes by foot from Syntagma square; you'll pass by a number of great cafés, but I suggest stopping at Cultivos. The apartment has everything you need and Ricardo welcomes you with some local food selections that won't disappoint.
Close by, there is Bayard café, which has a very cool and welcoming vibe; and for groceries, you're about 5 minutes away from a supermarket that has everything you'll need.