We would like to find a walking guide in Athens and some good day tours outside the city.
Your could get a copy of RS "Greece, Athens & The Peloponnese" Guide Book. It can be ordered elsewhere on this web site. I got a copy and it is full of very useful information on Athens.
Are you looking for a guide book or a guide person?
My understanding is that you can download the walking guides listed here: http://www.ricksteves.com/europe/greece under "listen" -- Greece Audio Tours and that they are free. You will need an app to do it and the info on that is here: http://www.ricksteves.com/watch-read-listen/audio/audio-europe.
I was trying to find this for you: www.greektravel.com. It's Matt Barrett's website and a RS recommendation. He has lots of info.