Basically look at Google Maps, find swathes of green with twisty white lines through it. Use Street View to make sure it's a sealed road.
Personally I would hug the Belgian border as far down as Luxembourg, head down via the Ballons des Vosges national park (google Route des Cretes), into Switzerland for the Grimsel-Susten-Furkapass triangle. Down to Lugano or, if you don't want to eat the cost of a Swiss vignette, east to Chur and either route 28 via Davos or down into Italy, across the Dolomites - really just stay off the autopista and choose any SS-road that looks nice on a map. From Tolmezzo, take SS13 to SS54, you'll end up pretty much at the south end of the Vrsic Pass. Spend as much time as you want in Triglav National Park.
Ljubljana is skippable, Maribor is lovely.
On the way back, assuming you're starting from around Maribor, the easy and fast way is via Graz to Salzburg - Munich - Stuttgart, but I would detour to around Lienz and do the Grossglockner south-to-north. If you're aiming for a ferry from Holland, then you're well-positioned to stop in at the Nürburgring on your way back; the Rursee in Eifel National Park is also quite nice.
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