If you want to go to one churchwhile you're there (and you should, then go to Santa Croce, but cunningly sneak in the back way to avoid paying €5 entry fees – go round the left of the façade and through the arch to the Leather School.  Follow the path straight ahead and go up a rickety iron staircase (this may sound perilous, but I’m saving you money so you can splash out on more ice creams later).  This is the entrance to the Leather school.  Go straight and you’ll hit the workbenches of the monks (they will gild initials onto anything you buy there for free) and you’re IN!  Ignore the signs saying “You should have paid to be in this bit of the church”, feel extremely clever and look innocent.  (No one checks, don’t worry.) 

Here you can see Michelangelo's vile tomb, a stunning chapel by the first artist of the Renaissance, Giotto, some beautiful cloisters (great for reading in on a summer's day!) and a brilliant Donatello too.  NB the markings just above head-height on the walls - that's where the water got up to in the great flood of 1966!  LOTS of artwork got ruined (especially everything in the crypts of course) but luckily the angeli del fango (angels of the mud) stepped in JUST in time to help with the clean-up operation and save some artistic greats.