from Viareggio, I'd recommend Versilia, Cinque Terre, Lunigiana and Garfagnana
Ciao Villaflood, sorry I had missed your post somehow so it has taken me a week to post some ideas for you....
Viareggio is a great base for visiting the Tuscan coast: the whole of the
Versilia as well as the
Cinque Terre but also perfect base for discovering
Lunigiana and
Garfagnana. I'd recommend you head inwards towards the Apuan Alps if you also enjoying walking and hiking - there are marked trails all over the alps to go up the peaks - for example, this one to
Monte Forato. You can also drive up from Lucca into the Garfagnana valley and visit Barga, Castelnuovo di Garfagnana, Bagni di Lucca.... I'd recommend going to see the
Wind Cave or the
Antro del Corchia (the first one is reachable from Garfagnana, the second from Versilia).
I also suggest you head toward
Lunigiana and
Pontremoli - the fortress there is beautiful, so is the museum housed within. This area of Tuscany is called the "land of the 100 castles", you'll see them on many hills.
Aulla also holds a fortress and it is also a museum but of natural life

, not what you'd expect. You can also head to
Carrara if you're interested in joining in with a tour of some of the marble quarries. Once you're in Viareggio, head to the local tourist office since they'll be able to give you brochures and summer hours for the tours.
Of course, you should make sure to go back to
Pisa on another day after you fly in.... you'll already by in Lucca and Florence so I'd suggest heading south and in toward
San Gimignano,
Volterra and
Siena on any other days..... I am giving you lots of possibilities, the ones I'd personally visit again, you won't be able to do everything but now you know which ones to consider! ;-)