Star Alliance has good Germany coverage. So I guess that would end up most likely meaning US miles for you.

Lots of flights to Frankfurt in particular. And from there you can either connect to other parts of Germany or take trains to get around. It's a big country with lots of stuff to do.

I would use either the UA or Aeroplan serach engines to look for available flights or use AwardNexus to do that.

I easily booked C class awards using UA miles on their website for flights on LH and OS from/to IAD for July. Lots of awards available for summer travel which surprised me a bit. You will need to call US if you go that route.