Ermou Street if a fun part of Athens if you need a break from all the (very interesting though) history.
Yes, it's a shopping street you could see in many other cities, but there are still some fun shops, some brands you will only find here. And it's a nice trip to start at the Akropolis, get down through the Plaka Stairs (and grab a bite there), walk through the Monastiraki Flea Market and finally head onto Ermou Street, ending up at the Syntagma Square. It's a fun walk. Be sure to stop quick to enjoy the wonderful Kapnikarea Church on Ermou.