I did a project last year that ended up as an exhibition and a Blurb book. It was documenting a local veterinary clinic and thoroughly enjoyed it. I got verbal permission from the people involved (staff, farmers and pet owners). Everybody was very friendly and helpful - only a couple of people didn't want me to use the photo's I took of them - so I didn't use them!
You can see the results here - http://sarphotography.wordpress.com/...tewartry-vets/
The best thing is to just go for it! Approach the cafe owners and tell them what you want to do and why. You could even suggest that they could hang some of the finished photographs on the wall of their establishments.