The Vosges region has good riding, look up "Grand Ballons". Its worth doing some of the B500 but it's heavily policed and has a LOT of 50kmh sections now as a result of people heading to it specifically. Ironic really as they built it as a drivers road to attract tourists. There are better, quieter roads in the area.
I've ridden along lake Constance twice, both times it was really busy with traffic. I'd actively avoid it now.
All negative points so far, sorry! But, You have plenty of time to plan. I find the UKGSER forum invaluable for this. They have a travel section listed by country. Also Best Biking Roads website is worth a look. Ride magazine have a route section with GPS files ready to download. Simon Weir books are worth checking out too.
I've tried various cross country routes through Belgium and only ever found lots of 30 zones and speed cameras. I just use the quickest fastest road through it now. Just about any non motorway road in Germany or France will be fun. Hardly any traffic (compared to the UK) and road surface will be perfect.
Have fun.
