I recently visited Ravenna just for one day. Luckily the trainstation is close to the city centre. Ravenna has been on my bucket list, mostly for it's historical importance, it was the last capital of the Western Roman Empire, it was the capital city of Theoderic the Great's empire and the capital of the Eastern Roman Empire's exarchate of Italy.
This city is one of the places which truly feel like a time machine, the down town is full with building built between the 5th and 7th century CE, and the rest is also very beautiful. I mostly focused on the mosaics, but I stumbled upon some hidden gems such as the palace of Theodoric the Great which is still standing and visitable from the inside.
The main attraction is Basilica di San Vitale, and I was truly amazed by it's mosaics, since I wasn't in a rush I could just sit down and enjoy it for a long time. A good tip is to buy the tickets for the main attractions before the trip online, since most attractions come with the same ticket and lines can be long during the day. I can't list every sight I've seen there but just a few examples are: Mausoleo di Galla Placidia, Battistero Neoniano, Battistero degli Ariani, Basilica di sant'Apollinare Nuovo and Mausoleo di Teodorico.
The city itself is much less touristy and this shows in the prices too. Relatively close to it are three very touristy places and somehow Ravenna is left out, but this adds to it's charm definietly.
I really recommend visiting it when you have the chance.