Cinque Terre Day trip possible?
 

We will be staying at an agriturismo in Massarosa - near Viareggio.

Thinking of taking a day trip to Cinque Terre. Driving up and taking a boat trip, stopping for lunch and a stroll and head back.

Is this doable or is it too much to squeeze in?
Recommendation on where to park the car?
Leaving in a few days!

It is definitely doable. Be aware that the boats don't run every day. We were there on a beautiful sunny day last July (no wind) but the boats were not running. If you can, hike the trails. We drove 3 hours from near Florence, hiked for 8 hours, and drove back. It was one of our favorite days in Italy! The views are beautiful. If you hike south to north, which you will since you're coming from Viareggio, the trails start out very easy and get progressively harder. I'm sure the views are stunning, albeit different, from the water looking up. You are so close by...definitely go!

Sure--that is not far. I prefer the boat from Portovenere, if they are running. If not, Riomaggore is the southern most and does have some parking.

Viareggio is a common stop on the N-S train route in Tuscany. I stopped by there going from the Cinque Terre to Lucca and also going from Firenze up to La Spezia. It's a classic, local Italian beach town to visit, with lots of little shops, cafes, stores, etc near the waterfront area. It is quite close to the Cinque Terre.

Riomaggiore, Manarola, Vernazza, and Monterosso al Mare all have public parking areas you pay to park at. As Bobthenavigator suggested, Riomaggiore has parking which I think is easily accessible. You could make Riomaggiore your base and then do the short (25 minute) Via Dell'Amore walk to Manarola, eat lunch in Manarola (Il Porticello near the water is my fav), then catch a boat in Manarola to the other 3 towns. It is entirely do-able and will be quite fun!

Great!

Will head to Portovenere (did a trip there last visit) and look to take the boat from there.

Are they easy to find?

Yes--hard to miss. Right smack in front of the big Grand Hotel. Good idea, but not if the seas are high.