driving to venice

If you can rearrange your trip to leave Venice for the beginning or the end of your itinerary, you will save money. If you start from Venice, start the rental after visiting Venice. If you finish in Venice, return the car when you get to Venice. If you need a car before and after Venice, you might even consider two separate rentals, if less expensive than keeping it parked for two days.
Parking is plentiful. There are lots of huge parking structures after crossing the bridge into Venice at either Piazzale Roma or Tronchetto island, next to it. However parking costs €25/day plus you are wasting money on the rental car because for two days it will stay parked gathering dust, that's why it's a good idea not to have a rental car while in Venice.

If you will take a minute to look at Rick's guide book, you will see that the place to park is the Tronchetto. It is an easy to get to multi deck parking garage that is right off the causway complete with a Vaporetto stop.

I would not worry about the car while it's parked at Tronchetto, which I used once do when I had a long-term car lease. Do pre-pack and cover whatever items you are leaving in the trunk (not valuables!!) before you arrive there. You can have the bags you're taking into town in the back seat. Arrival info is detailed at the front of Rick's chapters, e.g. p. 57 in the Italy book.

I've parked at Tronchetto also with no problem. We had to leave a trunk full for an extended villa stay so we backed our car right against the wall. Don't know if this made a bit of difference, but we left it for several days and didn't worry.

We also parked at Tronchetto with no problems. The drive from Milan to Venice is a bit hectic as they drive very aggressively and fast with trucks crowding the lanes. I would take a train to Venice if I were to go again as the traffic conditions made for a very tense drive. We've driven around Europe for years and this was my least favorite drive.