Take the train from Villa Literno - takes 2 hours through some great countryside and depending on what time you go it is only Euro 9.30 each way! The Trenitalia website is excellent (and in English), it will tell you which trains are the cheap ones. Beware though, tickets are not transferable and if you get on the wrong train, it could cost you quite a lot of money! Your car is pretty safe in the car park for the day (in fact, some of my friends leave theirs over night, with no issues...).