Flat and Italy is pretty much an oxymoron . If you fly into Malpensa, there is a very nice bike path all the way to Pavia. Very pretty. About 60 miles. You could then get a train to LaSpezia with maybe a stop at Genoa and or Cinque Terre and then back on the bikes and go down the coast to Viareggio. There is a bike path a good bit of the way. The traffic to get to the path is not all that bad.

Let me know if you need details.

There is also a bike path that is very flat along the Po River but I have not been there. It goes a fairly good distance, if I recall. The Po valley is probably the most flat area of Italy.