It is always hard for me to do restaurant recommendations in Italy because you can wander into pretty much anywhere and get a great, and I mean great, meal. For crying out loud, the rest stop gas station cafeterias make awesome to order pasta with a good glass of wine to go with.
When you are marching around Florence to see the sights you are going to get hungry. You may not want to stop for a perfect pizza (though you should) so for a quick and delicious repast I send you to I Due Fratellini or I Fratellini. This is a sandwich shop that has been serving up perfect sandwiches since 1875. I didn’t even know they had sandwiches then.
Pick from and assortment of wonderful panini made to order and get a glass of wine. Munch happily in the street wth the locals and few lucky tourists that have found this place. Swig your wine and leave your glass behind on the little shelves on the wall. Then march on!