Viareggio to Portofino--any advice for an inbetween stay

If you don't have a car already, while in Viareggio, I wouldn't bother renting one to go to Portofino. Just take a train to Santa Margherita Ligure (you might need a train change in La Spezia), then from Santa Margherita Ligure station take a bus or a taxi or a boat to Portofino. Actually, if you are reasonably fit for a long walk, it would be a very great walk (about 3.5 miles from SML station to Portofino).
Traffic from Santa Margherita to Portofino is often ridiculous (done it many times and it is sometimes faster to walk), the road is very narrow, and once there finding parking in the multistory garage is next to impossible (and it costs about 10 euro per hour to boot).
Along the way, if you go by train, you can stop at any of the 5 villages at the Cinque Terre or, if you have visited that already, Moneglia would be great too.
Train schedules and prices below:
www.trenitalia.com

While in Viareggio, pay a visit to Forte dei Marmi. That's where I used to hang out during my summers in youth.

A visit to the Carrara marble quarries would be interesting too. There are many companies doing excursions there.

Many thanks for the info-----we are staying in viareggio with friends and thought about renting a car to Portofino-however it sounds like a driving nightmare-------we wanted to take the car along the coast from Viareggio and up to santa margarita/Portofino---wondering if we could park in or stay over night in st marg?------moneglia looks great--------we have hopes of visiting the towns along the way-perhaps overnighting "inland"? and see bagni di lucca? husband and I have "hiked" the cinque years ago--------trip is sept 6/7----

wanted to see carrara-is a tour important? we also plan on stopping along to see forte die marmi--(too many places )
thanks
annetta

A car is not really needed for the Riviera, especially the Riviera Di Levante (Eastern Riviera, i.e. from Spezia to Genoa). The train is more convenient and less a hassle.
You are clearly very excited about your vacation, but hold your pants. You can't visit all those places in a day trip.