cruising from the port in venice
Getting to the port from the train station in Venice is easiest and most expensive if you take a water taxi. You can also walk over the pedestrian bridge to the right of the train station (though it has little steps so dragging luggage up it is a huge pain) and either get on a bus or take the People Mover to the port. Sometimes cruise companies will have free shuttle buses but they are not well-marked. The People Mover costs and you buy the ticket at the automated machine. To get through the gate, just scan your ticket and the little door will open so you can walk through. Be quick though! It does not stay open for long. When getting off a cruise, there will be mad rush for the People Mover and it will take quite a while to get back to the train station. Also beware of people who do not move away from the turnstile after they get through. I could prevent you from moving through and the door may shut before you have a chance to advance enough and then you'll have to buy another ticket.
Once you arrive at the port, the actual location of your cruise ship may be hard to find. There is an electronic sign showing the numbers of where the ships are docked, but there is no map showing where those numbered docks are! The Royal Caribbean ship for my cruise was docked way over on the far left side of the port and it was quite a long walk once I figured out how to get over there.
After exiting baggage carousel, follow the signs for autonoleggio to your left. All of the car rental companies are in this area on the ground floor. When you get the keys for your car, you will most likely also receive the paper showing damage already done to the car. It is very important to check the car yourself when you get to the parking lot because most of the rental companies have no employees in the lot to check over the car with you before you leave. If you notice a scratch or something on the car that is not marked on the paper, you have to return to the airport and get the paper changed. The parking lot is very small and often very full of cars, so also be careful when pulling in and out of the parking spot. My experience with Italian car rental companies has not been good, and they will try to make you pay for damage that you did not do. (As a side note, be very wary of renting from Locauto!)
Make sure you receive a little card to put in the machine in order to lift the bar at the exit of the parking lot. Once again, if you are out in the parking lot and realize that you do not have the exit card, you must go all the way back into the airport to get one. When exiting the parking lot, turn left to get to the roundabout and go right to head towards the highway. The entrance is quite close to the airport.
Gas station: When returning the car, there is a gas station directly across from the parking lot. However, this gas station is self-service and requires either Italian debit cards or cash. There is only one machine for payment, which is connected to all of the pumps. The instructions on the screen are in English, and you need to choose your pump number and put in the bills beforehand in order to be able to pump the gas. If you have larger bills and put in more money than what you can pump into the car, then you will need to take the receipt to the little building for a refund. If the building is closed (after hours or on a Sunday, for example), then you cannot get a refund. We had to waste 5€ because we only had 20€ bills, yet only 35€ worth of gas would fit into the tank and it was Sunday so the building was closed. Try to have smaller bills ready or stop at a gas station further away from the airport that is not self-service.
Italian highway: There are frequent tollbooths on the Italian highway and you can pay with cash or debit cards. Usually you take a card when entering the highway and then insert the card in the machine upon exit and it tells you how much to pay. I'm not quite sure if American credit cards will work in the machines as they usually require European cards with the chip. You can check the toll prices on the Autostrada website.
Vignette system: If you plan on driving on highways in neighboring countries, be aware that Switzerland, Austria and Slovenia all require their own vignette sticker on the car's windshield. You can buy this vignette at the border or sometimes in gas stations or even online. Switzerland only has the 14 month vignette (December 1 of preceding year until January 31 of following year) for 30€, but Austria and Slovenia have cheaper weekly to 10 day vignettes (8-15€) for tourists who won't be spending a lot of time in these countries. Sometimes rental cars will already have the stickers depending on where previous renters drove it. If you do not want to spend the money on the vignettes, then make sure to avoid the highways in these countries.
Leaving car in another country: Even though European countries are rather small and it may be easier to drop off the car in a different country if you are close to the border anyway, do not do it because the "abandonment" fees are quite high. Because of an emergency situation (Easyjet canceled our flight and stranded us overnight at the Venice airport with our 78 year-old grandmother so we had to drive all the way to Lyon, France), we rented a car between Venice and Turin, and then another car in Turin to be dropped off in Lyon. The fee for leaving an Italian car in France was 350€ even though Lyon is closer to Turin than Venice. The actual rental cost was only about 90€. If you cannot take the car back to the original agency, try to drop it off near the border and rent a different car when you get across the border (using public transportation or a taxi.)