Parking / Hotel advice (Venice)

Hello everyone,
I have a meeting in Padova this coming Thursday morning, and have taken a hotel in Mestre so that I can visit Venice with my wife on Friday at least. Given that I'm with my car, is it easiest to simply park at the end of the causeway and take a vaporetto around to the city.... or should we leave the car in Mestre, and take the train in?
We have no hotel booked for the Friday night, and would have liked at least one night in the city proper - any good suggestions below €100 will be gratefully received!
Cheers,
Andy + Claudia