I've been to Germany several times. They are keen bikers and you always get a good welcome at hotels & B&Bs. No skanky flea ridden places either, even cheaper priced places are good and you nearly always get a nice breakfast.
You've got the whole area from SE of Aachen through the Eifel area, down through the Moselle area around Cochem, Saarland, a flat boring bit over the Rhine valley and then you are into the Black Forest all the way down to Lake Constance.
I've been to Sachsenring 3 times and Lausitzring twice and have found some good roads on route. If you pencil off the area bordered by Cologne, Arnsberg, Paderborn, Harz mountains, Efurt, Bamberg, Wurtzburg, Mainz, Koblenz, Bonn, you'll find some nice countryside and good roads. Just stay off the autobahns and major roads unless you need to get somewhere quickly. Ive passed through several routes through that area on my way to the races.
The area around Munich is very flat and uninspiring on a bike.
I've also been down the CZ border area from Bayreuth to Passau which is good. Last year I went down there and continued to Linz then south through central Austria to Slovenia. The west and south west area in Slovenia I wanted to go to was hit by a storm so I avoided that and went futher south then east. It was ok, but not great. Then back north to Vienna through the eastern side of Austria.
I've also been to Austria before on my way to Brno in CZ and have found the Alpes on the western side around Landeck/Innsbruck/Zell am See to be dramatic in scenery but wet. The rest of the country is still hilly and has better weather. There also seems less traffic on the roads if you stay away from the bigger cities.
Austria is also good for VFM hotels and B&Bs away from the bigger cities which are a bit pricier.
The parts of CZ which border Austria and Germany are also nice areas, the towns tend to have an old historical city and a newer concrete highrise flats town area. Sometimes you arrive at a town through the concrete jungle and think this is a shit place, then go through the old town walls and its a different place completely. Ceske Krumlov in particular is worth visiting.
The Belgium Ardennes area is worth a visit. Some roads are bumpy through.
I've also been through northern Luxembourg a few times. Petrol is cheap as Luxembourgs have low tax on it, 30 cents/litre cheaper. You know when you reach the border as there are 10's of pertrol stations in a row for the Belgium/Germans to cross the border and fill up.
I've always left deciding the actual route to the day before as the weather can be variable. Going to Slovenia last year I went the way I was intended to come back, and came back the way I was going to go, sort of. I've always found hotels en-route rather than pre-book for that reason. The only time I book in advance is for a race meeting. I've got caught out once in the Harz where there was a huge craft fair and everywhere was booked for miles. The 1st hotel I asked at said best go 30k away. Another time in Germany at touristy Altenahr in the Eifel it took a 7 or 8 hotels/pensions to get a place. I usually go outside of school holidays.
Short trip to Belgium Ardennes, Eifel/Westerwald and back through Luxembourg last year in April.
http://www.photobox.co.uk/slideshow?album_id=759028776Slovenia/Austria/Gemany trip
http://www.photobox.co.uk/slideshow?album_id=803948169Nurburgring for WSB then a meander to the Black Forest then north to Edersee where the weather forecast looked ok
http://www.photobox.co.uk/slideshow?album_id=891822444 If anones got any info on Slovakia I'd appreciate it. Looking on the map the northern half looks interesting.