OK, so we went UK -> Reims -> Nancy -> Colmar -> Freiburg / Black Forest -> Heidelberg -> Cologne -> Gent -> UK. 10 days, 9 nights. Spent 3 nights in the Black Forest, and 2 in Gent. Stayed in a mix of AirBnBs and hotels. We went to these places cos we wanted to keep the driving down to at most 3 hours a day, and usually 2. Usually got going about 11 and to the new place by about 2.
We didn't really know anything about any of these places before we went there, but there was pretty much always something going on. For example in Freiburg they have a huge open air farmer's market on a saturday morning and you can get "Europe's longest cable car" from the mountain (Schauinland) where we were staying down into the town. Reim and Cologne have amazing cathedrals. The square in Nancy is stunning (Place Stanislas) and they have a sound & light show every night in the summer, which is brilliant.
We did the boat tour in Gent and also took a kayak out onto the river, which was a laugh. Belgian waffles are always good and we went to a great little pub on one of the evenings which my 12 year old daughter liked cos there was a picture of boobs on the wall. (
Trollekelder). Gent's a great place, like Amsterdam but without all the twats, would recommend it.
Driving wise, everything was great. The French motorways were pretty much empty. We had that badge thing that somebody mentioned before, that was really good. Think it cost about 30 euros in tolls. Autobahn was fine, just like a normal motorway but people were doing 100 in the fast lane. Cologne requires the clean air thing but is really easy to get into and out of. The only thing to think about is Gent - they do a park and ride but the one we used had no ride. It was just a normal bus into town. We had booked an early train back, so we moved the car into a car park in town on the last night. That was loads cheaper then leaving it there all the time (like 6 euros instead of 60). They also have their own clean air licence thing. There really are very few cars in the centre.
Oh, and you can use tesco vouchers to pay for the Euro Tunnel, and they count double, so it makes it loads cheaper. Petrol is also generally cheaper, but not by loads.
I'd recommend it. Even the kids seemed to like it.