From the electric train station of this name, heading west in the area that skirts the park around the Thission and Ancient Agora, at the end of the shopping street Ermou. There is the flea market where items ranging from off-brand winter jackets, to army knives, vie with old telephone cards (especially along Adrianou - which also has several rather romantic cafes and restaurants - on Sunday mornings).
Many gift shops and most notably antique, junk and collectors corners of every sort. This is touristy, but it's mixed in with the sort of old shops, that are gems and one of a kind.
By the way, walk just down from the station away from the flea market to Kirikou street and you'll find the busiest and most popular after-work Gyros places - if a Gyros is good anywhere, it is here!