The best and cheapest way is to leave the car in the parking lot in Mestre (this is the continental part of Venice). Parking in the Touring car park, next to the train station, costs only 7 EUR per day

On Venice Island there are only 2 parking spaces and the cost is much different from the parking prices in Mestre. The first parking is Piazzale Roma, the cost of parking is 38 EUR per day. The second one is Tronchetto, the cost of parking is 21 EUR per day