Florence has many beautiful gardens, here are 2 that I've been to and really enjoyed.
The Bardini Gardens and the Boboli Gardens.
Now, the Boboli Gardens are quite famous while the Bardini Gardens aren't as well known. I first went to the Bardini Gardens and there I was able to get a ticket for 7 euro which included entry into the Bardini Gardens and the Boboli Gardens. These gardens are simply beautiful and lovely to just wander around for half a day. You can easily spend more time but if you are looking for a relaxing morning or afternoon these gardens are great places to enjoy the nature beauty Florence has to offer.