Unless you are really running low on souvenirs emblazoned with male genitalia (and who isn't?) and/or non-descript leather bags, there's no reason to allow a lot of time here. It's a tourist trap and a dull one at that. Rub the nose of Il Porcellino for luck and kicks and be done with it.
you can't visit florence without stopping by this huge open market. I recommend picking up some scarves-- they are super soft and cheap! If you get a chance, go inside to the food market and have the best cheese and bread of your life
The pantheon feels most special when you get to experience with few people around... There's no lines, but there are people here all the time so it's another one of the places that you might consider checking out early in the day...
DO NOT MISS THIS!!!! This is an easy day trip and one of the most incredible places you'll ever see in your life. Try to get there in spring or early summer when everything is really at its peak. I don't even really like Monet's work that much but I come here every time I'm in Paris.