My self and 3 sisters, ages 50-60, are coming to Florence on a day trip between the 13th and the 18th september. Arriving from Arezzo to Santa Maria Novella train station. We will be mostly walking so we would welcome any information or help to make it a great day out.
I think you'll have a wonderful day in Florence no matter what you do. If you're not particularly interested in visiting museums all day, I'd actually recommend to just walk around the historical center and enjoy the views that Florence offers.
From the SMN train station, walk across the square and visit the Church of Santa Maria Novella. You have to pay to see the inside frescoes but I think they're worth it. After that, continue toward the Duomo - visit the inside of the cathedral and the Baptistery across the way.
Take Via Calzaiuoli and walk to Piazza della Signoria. Here you can enjoy the outside view of Palazzo Vecchio, Florence's city hall for over 600 centuries, the statues outside - the David has been this position since the 15th century although what you see now is a copy - then on to Ponte Vecchio. I am sure you will love the view. Eat great gelato at Grom near the duomo (one street after the duomo as you go on to via Calzaiuoli, turn left - generally has a line outside). If you want really traditional local cuisine, for lunch go to Fiaschetteria Nuvoli on Piazza dell'Olio (one street from piazza del duomo back toward the train station). It seems like a really small sandwich shop from the outside but has a small cellar downstairs where you can enjoy the day's menu of traditional Florentine dishes. I've always found the food to be delicious and at a great value.
If you feel up to the climb, you can head to Piazzale Michelangelo to get a great view over Florence. It is especially beautiful around sunset. You can see pictures of all of Florence's main highlights at Florence Pictures or learn more about Florence's main attractions at Visit Florence.