If you are not a confident driver, then a train anywhere would be your best option.
If it's all freeway driving (and you stay out of that far left lane) you're probably ok because it's a straight shot on the freeway into
Venice where the car returns are, but what fun is that?
Exploring small towns along the way is fun--just be careful of those skinny, winding roads--you may find yourself having to back all the way out!!.
To me, a rental car is just not worth the stress and hassle unless you're into exploring the small towns and back roads, and for that, I think you need to be quite a good driver.
Driving in
Italy is not for the faint-hearted!
Take good care!