Perche

My vote is not doable. You can't really count the first day, so that means you have 7 days. Since your last night will be at an airport hotel, you might not even count that night, so you have six days, and it is Milan, Cinque Terre, Pisa, Florence, Siena, driving through rural Tuscany, Bologna, Milan. You can physically transport your body to 8 places in 7 days, but when are you going to see and experience Italy? As JMN57 said, all you will be doing is packing and unpacking, and hauling yourself from one place to another.

Except for certain rural areas, for example in Piemonte, Valle d'Aosta, Alto Adige, Le Marche, Tuscany (sorry if I left any out), it is not anymore romantic or scenic than driving in northern New Jersey, southern New York, or the Central Valley of California. If you are hustling from one city to the next, you are just going to be on a highway, just like any other highway. Depending on the day and time of departure, Italy is quite a traffic nightmare. I'm talking about Milan. It's worse than LA, SF, or NYC. There is not much point in renting a car to drive from Milan to the Cinque Terre. It's a 3 hour at least ride on ugly toll road highways, then in CT there is almost no place to park. Sometimes, you can find parking in Monterosso, depending on where you are staying, but that is the least desirable of the 5 towns at that time of the year because you probably won't be able to use the beach, which is it's main attraction. Two other towns (Corniglia?) and one other have parking lots, and then it's a fair walk to the town. The parking lots are not in the towns. You will be dragging your luggage quite a distance, generally up some substantial hills. You can't use your car in CT, and parking is expensive. I'm going to guess 25 euros or more per day. Also, although probably not when you are going, the CT parking lots are small, and thus frequently full.

By train, Milan to Cinque Terre is a bit of a nightmare too. Figure on at least three and a half hours, and not on one of Italy's fabulous high speed trains, but on slow, old, stop every few miles, regional or local trains. You will surely have to make a train change at least once, and those trains don't come every 10 minutes, probably in Levanto, and wait a half an hour or so. It's probably better than renting a car, but it's not a good way to start a trip.

You might want to rethink Milan to CT. I agree, you are arriving in Milan too late to go anywhere else, and will be "stuck" in Milan the first day, so I wouldn't count it as one day in Italy.

Since your itinerary is more Florence/Tuscany based, It might be a better idea to go from Milan to Florence on a high speed train that will only take a little over a hour and a half, with only two stops, one being Bologna (don't get off the train). I'm not sure why you would choose Florence, but it's your trip. CT is more accesible from Florence than from Milan. You cannot have a car in Florence. Only someone with resident license plates are allowed, and there are cameras everywhere to detect non-resident license plates. And you will get a ticket in the mail, if not a charge on your credit card. The ticket might not come for months, during which time the amount will keep going up, even though you didn't yet receive the ticket. And if you don't pay, they will put a USA credit collection agency on you. No cars in Florence. It's doable, if you know precisely which streets you can drive on. It's doable, if you give your hotel information about your car, and they forward it to the police in time, but there is basically no driving in Florence. You can go more towards the outskirts by train or taxi, and pick up and return a car there, otherwise, you might wind up with major, major tickets. I've known someone who just got lost trying to find her way around, and kept circling and circling down ZTL streets trying to find it, and wound up with close to a thousand dollars in tickets just trying to find her hotel.

Like many places in Italy, no cars allowed in the center of Siena either, unless you are a resident. You have to park outside of the center, and take a significant walk into town. For that, you might just as well take the train from Florence, rather than just leave your car parked in the outer parts of Siena for two days. It's a fairly decent walk from the train station to the historic center, much of it uphill, but you'd have the same problem with a car.

Driving through rural Tuscany requires a car that you should pick up and drop off at carefully selected places, and know how to get out of town without getting a photographically generated ticket. Florence would be your base.

I just noted that you said you are staying at the Sheraton in Siena. Fatal mistake to your trip. Absolutely fatal. You have to remember that there are historic centers of cities, and that is what you go to these cities to see. The physical boundaries, due to suburban growth, can expand the boundaries of a city like Siena for miles. Piazza del Campo is the historic center of Siena, and it's where people go there to be. The Sheraton is almost an hour walk away from anywhere you would want to be in Siena. It's for corporate meetings, and things like that. You won't even be able to find your way to historic Siena from there. The way is so complicated that your eyes, glued to your GPS won't even help you. Don't go to Siena if you are staying in the Sheraton. If you are going to Siena, you have to stay in Siena proper, not in an outer suburb, on the other side of a major freeway, an hour away from Siena. This would be the biggest mistake of your whole itinerary. When you get to the Sheraton in Siena, you will be the most disappointed traveler in all of Italy. It is just not possible to walk from the Sheraton to the part of Siena you want to see.

If you have a car, I guess you can travel from the Sheraton Siena to actual Siena, but the Sheraton Siena is a half an hour drive away from Siena, and there are at least, minimum, 25 different street and highways turns, and no possibility you will not be lost because the streets don't have signs. If you walk outside of the Sheraton Siena you are basically on something like a highway, almost an hour walk away from Siena. Don't do it.