Need advice---Day trip from Florence

All you mention are fine, I cannot pick one over the other. Don't forget Lucca, which is on the way to 5 Terre. Also, Montecatini Alto and Montecatini Termed are cute.

We used tour company Walk about Florence which offers a "Best of Tuscany" tour. They take you in a bus to Siena, San Gimignano and Pisa, with a stop at an organic winery for lunch. It is a long 12 hour day but very worth it. It was one of the highlights of our trip.

With only 3 days in Florence, I'd advise allowing more time to see the city and the many wonderful sights it has to offer. If you are really not interested in seeing Florence itself, then you'd be better off staying in one of the outlying towns in the countryside.

Tours by Roberto does a wonderful day trip in Chianti: wine, history, scenery.

With 3 days in Florence, you may have just one day for a day trip. Florence itself is a treasure trove of Renaissance art. So unless you skip the major museums in Florence you don't have much time for day trips.
My choice for day trips would be
1. Siena - my favourite Tuscan hill town and fully deserves a day ( though staying overnight makes it more magical) . Well connected by bus from Florence which leaves you right in the middle of the old town, unlike the train which leaves you at the base of the hill. All sightseeing can be done by yourself with a good guide book. Great cathedral, the best piazza in Italy and the best view in my opinion from the bell tower.
2. Lucca and Pisa - Pisa with its field of miracles is worth a few hours. And we'll connected by train from Florence. Again doable by yourself with a good guide book.
Though Lucca has no blockbuster sights it's a very pretty walled town and has less tourists as compared to Florence. We stayed overnight for 1 night and walking the walls in the morning with the locals was a experience i won't forget.
3. San Gimignano - Pocket sized and super touristy but very pretty all the same with its multiple towers. And has the best gelato prize winner as an added attraction.

Have a great trip!

Some people love those full day bus tour groups that take you to multiple locations all in one day.
Winery for lunch, multiple towns/sites throughout the day, etc...

While my preference is to avoid those like the plague if I can help it, just not a group tour person, if I were in an area, only had 1 day and felt I would never return I would be tempted.

My advice is to plan just a daytrip to Siena and back on public transit without a tour, no other towns, nothing else just beautiful Siena, it will offer plenty to do for a day and not leave you totally exhausted considering Florence itself will occupy your other days which will keep you busy walking and tire you out plenty.

I used Viator.com to book a winery tour in the Tuscan countryside. They take you on a coach bus and bring you back. I also used them to book a cooking class in Florence, which was fun. If you are only going to have three full day sin Florence, though, I would do one day in Florence, one day on a day trip to somewhere like Siena, and then the third day in Florence again (maybe taking a cooking class). Back to back day trips outside Florence are exhausting and you will miss out on what Florence has to offer.

-Siena
-San Gimignano
The above can be done both in a day, but it's a bit rushed without a car.
-Chianti Hills
-Lucca+Pisa. do Lucca first, which is prettier, then leave a couple of hours for Pisa, trains back to Florence from Pisa are frequent (nearly 60 a day) until late night.
-Cortona+Arezzo

Each is a day trip. With 4 nights in Florence you can do one or possibly two day trips. A couple of days are needed for Florence.

Personally, I would skip the tours as well as so many day trips are so easily done by public transit from Florence. Going solo will allow you to sightsee as long or as little as you choose to or scrap the plan entirely if the weather doesn't cooperate. But then, we also avoid tours as we're so much happier not being locked into a set schedule/itinerary.

Siena (by bus)
Lucca +Pisa (by train)
Fiesole (a very short trip by Florence city bus)
Etc.

But I'd agree: with the time that you have, 1 day trip is enough. Florence offers PLENTY to see to fill the rest!