Go for a stroll by the water's edge and enjoy some fresh air at Viareggio Harbor. It's always nice watching the boats coming and going.
You'll have no end of sightseeing choices when in Viareggio. Viareggio Harbor is a great place to start here, but there are loads of other interesting places to consider. Massaciuccoli Lake, Pontile di Lido di Camaiore, and La Cittadella del Carnevale are close to the place for ships to anchor and will top off your day nicely.