I'm in love since I was a child with the Duomo of Santa Maria del Fiore, but also Santa Croce is a beatiful church! I had no idea there were so many parts to explore beside the church itself!

You should also consider visiting some famous gardens, like Boboli, which is close to Ponte Vecchio and Palazzo Pitti!
The are also some free ones, like the Giardino delle Rose (close to Villa Bardini) and the Giardino degli Iris (close to Piazzale Michelangiolo... you can enjoy a beautiful view of Florence from there!), but probably this is not the best time to visit them, as both roses and iris aren't in bloom right now.

Hope you are also going to visit the Uffizi... if you love art, you'll be amazed by that museum and you can't miss it! :)