Well, Pisa is easy to get into...I don't want to remember the fact that it took us 1/2 hr. to figure out all the one way st's. (not listed on the map!) to get OUT!

I'd choose Siena, and I second Lucca as my day trips. Why? well, I didn't find Pisa ugly, just boring after the tower and via dei Miracoli (it does have some of the best kitsch souveniers on the planet!). Met some nice folks...

Siena, now Siena is very red/brown brick (hence the color burnt siena-yea, I know!). A huge gracious central campo, neat churches, some cool museums, excellent restaurants, a great market. Plus there's the whole contrada/neighborhood thing that lends it extra charm.

Lucca is also wonderful for many of the exact same reasons, plus the wall. Rent a clunker bike and roll around there, or just stroll during the evening passagiata and you'll become a true convert. Some great gelato too!

Now about Florence. A lot of folks have really been down on Florence--it's a bit of shock between Rome and Venice. Venice is relatively quiet and has tons of novelty (and I love it).
Rome-well, generally you're talking wide streets, and a huge old old city (also love it!). But Florence is medieval/renaissance; tiny sidewalks and streets and high buildings (well, 4-5 stories) that make reverberating chambers when the legion of motorbikes stream by. I adore it. It's just not a great place to be when it is really crowded or really hot. That's why I tend to stay OltaArno or out at Pensione Enza where it's quieter and more residential. Also, they've closed off more streets to traffic in the center, so it's quieter there.
Anyway, if you choose Pisa, and the charm is done after a few hours, head back and stop in Lucca or Montecatini off the autostrada. They're very close by.

Rented a car last summer in Florence and saw Pisa "Field of Miracles", San Gimignano, and Siena in a single (long) day. In Siena,the last stop, we ran out of time and only had time to walk to the city center and have pizza on the Campo as the sun was going down. Siena is the larger and more interesting and significant city of the three. If I were to do it again as a day trip, I would skip San Gimignano (even though it is lovely) and just do Pisa and Siena.
[We returned to Siena a couple of days later for an overnight stay and more comprehensive tour.]

I don't think anyone goes to Pisa to see anything else BUT the tower, do they?

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I find Siena much more charming. However Pisa does have the tower and if you want to visit one of the world's most recognized tourist attractions, then head to Pisa. As to whether or not seeing & trekking up the tower is worthwhile, IMO, it's a "class b" site. Not a must-see for a first visit to Italy; however if you're in the neighborhood, then sure, why not, do the Pisa thing.

A recent visitor posted helpful comments ---
PISA: Invaluable Quick Tips >> http://www.fodors.com/forums/threads...p;tid=34417879

We had the same dilemma but,following my friends's suggestions,we visited Pisa in the morning and Siena in the afternoon.We had lunch in San Gimignano.
We used Mr.Camilli,a native of Florence not licensed guide but good and prompt driver(sometimes crazy..)His car was roomy and with AC.We also hired him for our day trip to Cinque Terra.His email is [email protected]

From Florence:
To Pisa by train. One hour. Many trains until late in the night, in the hour. The Piazza dei Miracoli is one of the prettiest in all Europe.
To Siena by car. Visit the Chianti region, San Gimignano, Montalcino, Pienza and Montepulciano.
I'd rather stay one or two nightas in Siena for a complete visit and trip around it.
We rentend a villa on a XVII manor house.
If interested give me a note.
Ismael