How to get from Venice to the airport (and from the mainland to Venice the day before)

There is a boat that takes you to the airport. We picked it up at San Marco and from there it takes at least an hour because it was not direct and made several stops. You would definitely need to check timetables and make sure to leave an appropriate amount of time. The other boat option would be a water taxi which is more expensive but maybe not too bad depending on your group size.

There are no trains from Venice to the airport, but there is a public bus that runs from Piazzale Roma and stops near the airport hotel, but I am not sure if it goes to the airport. One option is that you could stay at the airport hotel and then use the bus to go back into Venice and take the last bus back. The airport has a shuttle van that will take you to the airport in the morning. My husband and I did that but we had already spent several nights actually in Venice.

Additionally, if you do decide to drive and park the car in Venice before taking it the airport, people have mentioned that they got speeding tickets for barely (1 mph) exceeding the bridge speed limit. Be careful!

Stay in Venice, by all means. There is a large parking garage (several actually) off Piazzale Roma if you want to hang on to the car. There are lots of hotels walking distance that will have all the character of other Venetian hotels, ie on a canal. As was mentioned, watch your speed on the bridge.

I've taken the bus directly to the airport. If I remember correctly, it doesn't make any other stops and was not an hour drive.

Check http://europeforvisitors.com/venice/articles/vi-transport.htm for a thorough orientation of your options.

We took the water taxi once we got to the central train station (it was a little confusing at first) but we stayed at Ca' dei Dogi which is right down a small walk way from the San Zaccarrin water taxi stop (on the Grand Canal) and right behind the Dodges Palace but a world away from the hustle and bustle of San Marco square even though just a walk around the corner. We loved everything from the breakfast, great staff, easy to get around walking, and if you every get lost just follow signs to San Marco and the hotel is just around the corner. You walk down Calle deghi Albanesi and then of the left you see a little alley with a sign for Ca' dei Dogi, which opens up into a little court yard with tables. Go to trip advisor and read the reviews as I can't say enough about the hotel, location and friendly staff.

We had a room with a little patio facing the back of the Palace and every day we would come back from sightseeing and sit with a glass of wine and it was just so peaceful we kept saying we couldn't believe we were in the middle of Venice. Also for getting back to the airport, the water taxi for that is also right along the water and the staff will tell you where to go but it is very close and the water taxi takes you right to the airport. You can buy tickets in advance as there is a booth on the water.
I would say leave the rental car at the train station and take the train into Venice then take the water taxi to your hotel. Getting home you may want to book a private water taxi, they are the wooden boats and they would be waiting for you on the pier. At Ca' dei Dogi we had them book one as on the day we were to leave they had a Dragon Boat Race on the Grand Canal and none of the big water taxi's were allowed to run only the small water taxi's so we booked one, more expensive but very direct and if you have a group of people going it may cost less.

Water taxis are about 100+ EUR per trip between the airport and hotels in Venice. The parking garage at Piazzale Roma is pricey, and parking it at Mestre station might be an option. trains for Mestre to Sant Lucia on the island are pretty cheap, like 1+ EUR per person. If you stay in the Sant Lucia station or Piazzale Roma area, you don't need water taxis. Also taking the water taxi to the airport still leaves you with a 650 yard walk to get to the airport. The public water buses (vaporettos) are 7.50 EUR pp per ride.

Is your 10am flight a flight all the way home, or more likely, a short flight to another larger European city where you will connect to a big plane for the long flight home? Your profile doesn't point to which country you are from, nor do your previous posts.

I'll assume somewhere in North America, and if that is true likely it will be one of those short flights. If it is a short European flight you don't need to be at the airport 3 hours ahead which you would if it was a direct US bound flight. 2 hours should be fine.

For clarification of previous answers here in this thread I'd like to clarify a few things.

There is a bus which goes from Piazzale Roma near the train station in Venice does go to the airport in a very quick, frequent and very cheap way. They go right up to the door or the airport.

The boats which can be referred to as water buses but are called vaporetto call on specific routes and stops every few minutes around the the main part of Venice. You can get a day pass or single tickets but if you use the boats much at all the day pass is much cheaper. They don't go to the airport. They do connect the Piazzale Roma, the station, and stops which can be near your hotel in Venice and main tourist attractions.

The boats which go from certain scheduled vaporetto stops on a fixed schedule to the airport is called Alilaguna (colour) where colour indicates the route, like red, blue orange. They are much more expensive than the bus, much slower, and not particularly nice boats.

There are also very expensive private boats which you can hire to take just you and your party direct from the water dock nearest your hotel to a dock near the Alilaguna dock at the airport. They are very expensive and may or may not be faster than the bus depending on where you are staying. They are called water taxis, and like any taxi they can take you anywhere a taxi is permitted, as well as the airport.

Both the water taxi and the Alilaguna docks are about a 650 metre, 10 or 15 minute walk (with luggage) from the door to the airport building.