You have nine days away, from which I guess you deduct at least some time for travelling from home to Ijmuiden and back, even if a part is overnight on a ferry.
Let's say that you have seven full days, which is plenty of time but Germany is a huge country, so you won't see it all. You tell us that you are used to touring in France and Spain (both very big countries too) so you will no doubt have a good grip on how far places are from each other and the time taken. You should also have a good idea how many days you want to spend riding and how many you might want to spend off the bike and, not least, the type of roads you want to spend your time riding.
That just leaves you wondering where to go and scenic routes to ride to get there.... Just how long is that piece of string?
Let's start with areas....
You are coming from the north, so you could do something different and unusual and tour around the north German coast and into Denmark. Plenty to see and do.
Heading southwards, you could take in the Ardennes, the Eifel and then maybe head down further into the Black Forest and the French Vosges. That could take up your seven days and you may well have missed out the Rhine / Moselle (or not) depending on which way you went and chose to loiter.
Or you could skip a chunk and ride to the German and Austrian Alps and spend more (or less) time there.
Or any combination of the above you might be keen to dream up.
Roads to ride, one: If you have a Garmin GPS there are hundreds of suggested routes, lovingly created for just what you have asked for. Several of them sit two are three threads above yours. Open them up and play around, you can't break them.
Roads to ride, two: Have a look at the loads of trip reports, questions on where to go and how to get there. You ain't the first and you won't be the last.
Roads to ride, three: Look at your maps and see those that interest you, depending on where you plan on going.
Roads to ride, four: Google things like 'motorcycle touring in Germany' and see what pops up...
https://www.google.co.uk/search?q=m...many&ie=UTF-8&oe=UTF-8&hl=en-gb&client=safari
Dream up simple combinations of words involving Germany and motorcycle and roads, then pump them into Google. You want personal recommendations? That is what you will get, thousands upon thousands of them.
Some of the sites might well be organised tours but hey, you don't want those as you are an independent kinda' guy with your mate... Never mind, you can still have a look at where the tours go and steal some of their ideas.
Hotels to stay in, one: There are hundreds if not thousands of hotels in Germany, they will not all be full. But as you have no idea where you'll be, it's tricky to suggest one or two in particular.
Hotels to stay in, two: Google something like, motorcycle hotels in Germany
https://www.google.co.uk/search?q=m...many&ie=UTF-8&oe=UTF-8&hl=en-gb&client=safari
Or Motorrad hotels in Germany
https://www.google.co.uk/search?q=motorrad+hotels+germany&ie=UTF-8&oe=UTF-8&hl=en-gb&client=safari
Or biker friendly hotels in Germany
https://www.google.co.uk/search?q=b...many&ie=UTF-8&oe=UTF-8&hl=en-gb&client=safari
It really is a fantastic tool.
Last but not least....
IT IS YOUR HOLIDAY NOT OURS. Do some work with your mate you are going with. You are not without experience from past tours in Spain and France. Apply some imagination between each other. You'll do it over the weekend if you apply yourself to it....