Walking to town. It will really depend uopn where your ship actually ends up docking. If it is at the bottom pier in the referenced picture, then you should be able to walk. Any other locations you will be in the middle of a very busy port and they probably will not let you walk.
The shuttle bus to Piazza Grande should be free.
Radiance of the Seas - Nov 2006
Legend of the Seas - Jun 2007
Costa Fortuna - Mar 2008
Legend of the Seas - Oct 2008
Celebrity Constellation - July 2009
MSC Orchestra - Jan 2009
Navigator of the Seas - March 2009
MSC Poesia - November 2009
Jewel of the Seas - June 2010