fietsbob

If a bit ambitious, you can start in Rotteram and follow the Rhine up to its headwaters in Northern Switzerland, not far from Basel,
and not too far is the Drainage basin Divide , then you come to the headwaters of the Danube,
and you can follow that to its Delta on the Romanian Black Sea, coast

I rode a section north from Basel , and east from Linz to Passau on each river..
there are Levee top Roads to be used,
Passau to Vienna Is really Popular.

commercial campgrounds abound..

Of course in addition to Natural Rivers there are Navigation Canals to follow.