Perhaps the German approach is the best one for now. No testing at all for travellers from areas designated as safe on the basis of a set, transparent rolling number of cases. That's about enough of a stimulus for non-essential travellers to direct their attention there. If there's still someone who insists on going into a high-risk area, they need to either test negative before their return trip and present a time-stamped certificate at the German border - or they need to get tested at a designated test centre in Germany and self-isolate until such time as their result is out and negative.