I've been writing a blog, with a lot about Greek restaurants and food, but haven't gotten to street food yet.
Until I do, I recommend Thanassis kebab, its great!
souvlakia at Meraklides in Kolonos is also amazing. (dont mix up Kolonos with Kolonaki- two different places, although close in distance).
A lot of vendors in the center sell Kritsinia, which is sesame bread, and yummy.
Spinach pies and Cheese pies are also readily available.
In case you want to check my blog out though, its: http://www.emambrosia.wordpress.com
And, so far, I love what I see on your blog, Evie! Still looking for your review of Ta Kioupia!
Antman, here's a link to marketmanila. It's not specifically about Greece but the author did visit this year and write up a few casual meals. There be might some pictures of the Thanassis souvlaki etc., mentioned above by Evie. For some reason, there isn't much info on Athens here at Chowhound. Try Tripadvisor Athens. They are always talking about food!