We went to Florence for 4 days last June with 3 young adults 20, 19, 18. I would suggest booking tickets to the museums far in advance. The non ticket lines were very long.The city is very walkable with lots to do and places to eat. We loved the live music in the squares in the evenings and trying to find some of the wine windows throughout the city. Someone on here suggested this tour group
https://www.ciaoflorence.it/en and we took the full day tour to Sienna, San Gimignano, lunch with wine tasting, and Pisa tour. It was an amazing relaxed day. HTH.