Ravenna

Take the train from the train station in downtown Florence, "Firenze Santa Maria Novella." It's approx. 2 hours, with one change, from Firenze SMN to Ravenna. See the trenitalia.com website for dates and fares. (The website will convert to English if you check the little English flag in the upper right of the screen.) If you have an unlimited budget, the easiest way would be to book a private transfer; that would cost you multiple hundreds of euros.
Update: this was just posted in the Italy forum on 2-18: Posted by Frank (WPB, FL) on 02/18/19 03:58 PM

The Florentine, Florence's English language monthly, reports that the T2 tram line between the airport and SMN opened on February 11. The tram will run from 5am until after midnight, and the ride is scheduled to take just over 20 minutes. It appears a standard 1.50 euro transit ticket gets you to or from the airport (or you can ride for free until February 24).

Here's the website. https://www.gestramvia.com/orari

I didn't find a car necessary at all in Ravenna. All but one of the mosaic sites was in town. There was city-bus service out to the other one at the basilica in Classe. Now, I do walk a lot, probably an average of 6 miles or so a day when in Europe, so the distances involved might be more noticeable to some others.

Google Maps tells me the Basilica di Sant'Apollinare Nuovo is about 0.7 miles from the Mausoleo di Galla Placida, and the walking route takes you very near the Arian Baptistry and the Basilica of San Vitale. The Baptistry of Neon and the Archbishop's Chapel (very near each other) are less than 0.5 miles from Sant'Apollinare Nuovo in a different direction. Sant'Apollinare Nuovo is itself about 0.4 miles from the Ravenna train station, and the bus to Classe departs from very near the train station, basically on the way to/from St. Apollinare Nuovo.

For a day's visits to the most popular mosaics in Ravenna, no car was needed.

What I do remember about staying overnight in Ravenna was a lot of partying noise outside the hotel. This was in late summer and there apparently were beachgoers with loud motorcycles revving the engines at all hours. Not sure whether that is a common problem.

I can back up what acraven says. I got to spend four nights in Ravenna in 2015 and really enjoyed it. Walked everywhere except the one site mentioned. Stay in the town center.

Another who agrees that no car is needed. One can walk to all but one of the major mosaic sites. It was fun; like a treasure hunt. We also walked between the train station and our hotel.