If you like mosaics or early Christian monuments, the 2000+ year old city of Ravenna is a gem. Yesterday I wrote about the mosaics in Piazza Armerina which depicted daily life; the mosaics in Ravenna are all of a religious nature.Ravenna isn't currently a significant city in Italy from a political standpoint, but in the year 402 it became the capital of the Western Roman Empire so it's had a pretty interesting past.
The photo above is from the Basilica of S. Apollinare in Classe, built in the 6th century, and that whole image is made out of little colored cubes of glass. Amazing.
The reason mosaics are so spectacular even after 2000 years is because they are made of pieces of glass, so the color doesn't fade over time. It's impressive to think that the artist had to figure out exactly how many pieces of each color of glass were needed to create the (gigantic!) image, and then each color of glass had to be made (or in the case of gold, layered with gold leaf), then cut, and then carefully placed. Not exactly something you can do in a rush.
Mosaics are sort of an ancient pointilism, aren't they. Remember the scene from Ferris Bueller's Day Off, where the camera focuses in on one dot of paint on Seurat's Afternoon on la Grande Jatte, and pans out until you can see the whole painting? 
Logistics
You can get to Ravenna in a couple hours by train from Venice, wander its medieval streets, or visit the eight early Christian monuments on the World Heritage List.
Ravenna is not on a main train route so you'll probably have to change trains and get a slower one for the Ravenna leg, but it's worth it.
I'd definitely plan on at least one full day there, though it depends how many hours in a row you can stand to absorb art. I can pay attention for about two hours at a time so I'd probably try to spread it out over a couple days.
Also the sites are somewhat spread out, so either organize taxis, rent a car, or navigate the Italian public transportation. It's a pretty major tourist attraction for Italians so you can definitely get around to the sites easily.
And from a tourist point of view, it's great: most of the monuments are open 7 days a week until 7 pm, and not expensive (a ticket for 8.50 Euros gets you into the "top 5").
