Where is the best fairly level biking in Bavaria?
I believe there is a bike path along the Danube (Donau) river. I saw it in Vilshofen and I think it goes from there all the way to Vienna, but I'm not sure where it starts. The Donau starts in Donaueschingen, near Triberg in the Black Forest, and it goes from there to Donauwörth, but I don't remember seeing the path in Donauwörth. (ut, then, I wasn't looking for it, either) There is a whole chain of Gasthouses along the path. I stayed at one of them and they were surprised I didn't arrive on a bike. There might also be something along the canal the connects to Main River to the Donau.
Well, you didn't specify where in Bavaria, as it is the largest state in Germany by area... But to give you some research ideas, try a Google search of the word "fahrradwege" which translates to "bike trails". Here is just one site I found doing this: http://www.bayerninfo.de/vib/rad;jsessionid=91746E4A0E435D360F01A130B1FE8CF5 German book stores usually sell all kinds of guidebooks and maps for hiking and cycling. There's usually no English available, but the maps and graphics are easy to figure out. Germany is criss-crossed by an extensive network of well-marked cycling trails. You should be able to find more than enough routes to fit your needs.
A nice bike tour is around Lake Constance. It's sort of level (compared to the Alps) and half in Bavaria.
Check out www.crazyguyonabike.com
There are over a dozen trip reports (Journals) on riding the Danube River trail.