I agree that you could easily occupy yourself in Athens for a week given your interests. I vist every 6-8 weeks for 3-7 days at a time & I'm still finding new places, as well as not having done all that's on my list!
The Peloponnese as your other choice is excellent too (I live there!), so to give you some ideas of what to have on your list for that part of the trip here's some photos taken in the last few years,
http://www.flickr.com/photos/yiassas...7625881425523/
http://www.flickr.com/photos/yiassas...7626514959190/
http://www.flickr.com/photos/yiassas...7624647870023/
Sorry, there's a lot, but there are many places among them that you'll never see in guide books!

Yiassas -- thanks so much for the link to all the great photos!

No problem, if you want any more info or directions on the places included just ask!

as always, wonderful pics yiassas :)

Thankyou Clausar! Will be in Athens again in a weeks time, so will be adding to those sets probably!

Just a comment, if you are doing extended day trips like to the Corinth Canal from Athens, why not make it part of your trip to the Peloponnese? You don't need a tour to see it. Naplion is a great base. Mommovasia is a must see. Extra days in Athens can be spent going to the non touristy areas of Athens, such as Psirri, Kolonaki, Gazi areas outside of the Plaka.