Some interesting European routes using road bikes, although riding loaded with panniers.
In France we cycled along the Dordogne River in southern France. No signed bike route but lots of quiet and paved local roads.We also went along the nearby Lot River but it is quite hilly riding between. Very scenic but do expect some hills.
In Italy the Veneto area (the province around Venice is interesting especially in cities such as Padua, mostly flat and there are lots of quiet roads.
The Danube Bike Route from Passau (Germany) to Vienna (1 week) or continuing on to Budapest (2nd week) is interesting and works for road bikes with lots of cyclist friendly hotels along the way. The sections in Germany, Austria and Slovakia are well signed and easy to follow. At the Hungarian border you lose the signs so you need a guidebook. Only get hills when you get into the Hungarian wine region for a day, otherwise flat or very gently rolling on the route.
My favorite route is the Romantic Road which actually does not follow the tourist route by the same name but there is a well signed bike route. The bike route goes for 420 km from Wurzburg near Frankfurt to Fussen near the famous castles and Austrian border. Mostly paved, always well signed, I love the variety along this route. There are flat sections and and large gradual hills in sections but we used loaded road bikes staying in B & B or inexpensive hotels,
Tom Oxby