Germany, Czech, Austria in December (need advice)


I have a business trip to Munich in the beginning of December and I’ll have 2-3 spare days there.
Of course I’m thinking about short packrafting trip.

I’m looking for:
50-120 km (so it would be possible to make in 2-3 days),
as little portage (weirs and dams) as possible,
scenic environment,
preferably easy to paddle - Class1-Class2 (i don’t want to bring life jacket, helmet and wetsuit with me, though i’ll take them if the river will have Class3 sections).

Inn, Isar, Sazava, Morava,many others… Will be there enough water… Aren’t they frozen in the beginning of December… Too many questions…
What would be your choice?
Any recommendations are greatly appreciated!

Thanks, Kirill