I can definitely recommend Germany. Southern Germany is normally more weather-proof with the area around Bodensee/Lake Constance the best. Saying that we were there 3 years ago and had a week of rain and 17 degrees after one week of 25 plus.
You also have the lakes around Munich, medium mountain size in Frankonia north of Nurenberg with lots of hiking possibilities if you don't want to go totally alpine.
Palatine next to the French border is also lovely for hiking (and it is a great wine region, like Franconia as well). You can drive in around 7-8 hours there.
A bit further and you need an overnight stay is the Baltic coast. Either north of Hamburg the lakes around Eutin/Ploen or the beach resorts of Timmendorfer Strand and north of it or Mecklenburg-Vorpommern, island of Ruegen.
We have been several times in August there and normally have lovely weather with the odd days of bad weather thrown in. But DH and I are Germans, we go also to keep DD's German skills up and running, so for us it is like a UK holiday, we don't go there for sun proof weather.
You may have left it quite late though, we normally book in Autumn for the next year Summer, the German as well like to do things way in advance.
Another area worth checking is Austria, lots of large lakes used as beach resorts.