You know, "Northern Europe" is a very fuzzy term, particularly to a European. It's like saying North America. Be more specific about where you will be coming out of - Scandinavia? Germany?
Without knowing any details of your trip, I'd say that you can open the map, hit any mountainous area on it and have a blast of a time.
Really serious fun areas are within one day's ride from almost anywhere. The grand-daddy of all fun riding are the Alps, of course, particularly the Slovenia-Austria-Germany-Italy-Switzerland axis.
I also had pleasurable rides further east, in the Carpathians, but the accommodation base over there is not as developed. This is probably also away from your radius of interest.
Going west, the unheralded, but great riding areas just within a few hours from the North Sea are the Vosges range in western France and Massif Central in south-central. Vosges, in particular, is off most riders' radar - but it has great winding mountain roads and wonderful wine-country villages. I did a few 3-day jaunts there and enjoyed that thoroughly.
You can arrange a rental before-hand, in the US, through a touring provider like Ad-Mo Tours, http://www.rental-motorcycle.com/
, or Bosenberg, http://www.bosenberg.com/
. Both of these have rental stations in several EU countries. I've used both and had good experiences with them.
If you are going to be coming out of Germany, you are in luck: that is the best place to pick up a machine.
Beside the above agencies, I used those (both have English-speaking staff):
In Munich: MotoGreek Motorradvermietung
phn.: +49 (89) 7849904, email: [email protected]
contact: Stergios Papagatsios
In Wiesbaden, just 20 minutes from Frankfurt Airport:
Motorrad Center Tullius GmbH
phn: +49 (611) 182106-0
email: [email protected]
contact: Andreas Fehr
Below is a Web site that lists a whole network of participating renting Motorcycle Dealers, sorted by brand. Knowing German helps, but you can slog your way through and at the very least find contact info.
Your US license (with motorcycle endorsement!) generally suffices. Nevertheless, make sure to have an International Driving Permit - and make sure that the AAA clerk stamps not only "B" (cars) but also "A" category (motorcycles). Few of them know about it.
Even though they are supposed to, rental agencies rarely check IDP. However, I have been stopped at police checkpoints in Austria and asked for it.
Hope this helps.