sounds like a wonderful trip!
Ciao szarate,
A 25th anniversary celebration in Tuscany sounds wonderfully romantic, I am sure you will both love it!!
Have you been to Italy before? I think the plan you have of where to stay for most of your vacation already sounds really good, I endorse the 4 nights in
Florence so that you have enough time to enjoy it going at a good pace rather than trying to cram everything into less time. There is lots to see in Florence - from the cathedral, baptistery and bell tower (I suggest a climb up to the dome or up the bell tower, you get an amazing view of Florence from there - don't do it the first day, but rather after you've been around sightseeing at least a day so that you can enjoy picking out where you've already been, it is a lot of fun!), Piazza della Signoria, the Palazzo Vecchio, the Uffizi and Accademia as well as all the churches -- there is a lot to see and you also want to take the time to enjoy your meals, eat lots of gelato and enjoy being in Florence for such a special occasion.
You definitely don't need a car while in Florence but it would be really convenient once you leave into Tuscany - it would give you the freedom to move around as you please and not depend on bus schedules -- although there are some places easily reachable by train and you could do those before you get the car, such as
Pisa or Lucca.
The 7 days in Tuscany could be divided between a few days in the
Chianti area (2-3 days) so that you can visit the hilltowns in Chianti and do wine tastings. You should also visit
San Gimignano. This is all in the area between Florence and Siena.
The other 3-4 days you should move further south, to below
Siena. If you stay in Montepulciano you can easily visit the
Valdorcia towns on one day,
Cortona and Sansepolcro on another, Arezzo as well. Somewhere in between visit Siena too of course!
You could head west toward the seaside for at least 2-3 days in the
Maremma area near Grosseto before going up to Cinque Terre (2 days) which is no longer Tuscany but Liguria.
The reason I say this is so that you can enjoy the flat sandy beaches and beautiful wild landscape that Tuscany offers in this area. There are wonderful wine routes here and the "cities of tufo", the hilltowns sitting and carved into the tuft rock of the area as well as the beautiful National Park of the Maremma along the coast.
The Cinque Terre (literally a translation of "five lands") are on the rocky cliffs right on the water, no sandy beaches aside from a stretch in Monterosso and are very romantic but also expect crowds. They are no longer a "secret" and so many people go there sometimes you walk in line along the trails.
Hope you're enjoying planning your trip - let me know if you want any additional ideas!