The ruins of Fortress Hohentwiel, located in the south of Germany, are definitively worth a visit. You can easily spend several hours hiking up the hill and explore the complete area while reading the historical background of this former fortress. We recently dayhiked to the fortress starting in the town of Engen, north of the fortress.
There is a well-marked hiking trail called Querweg which connects the city of Freiburg with the Bodensee lake. One section (26 km) leads from Engen to Singen via the fortress and visits also some other, smaller ruins beforehand.
Although the weather was rather blurry, it is an amazing landscape which offers great nature and promising views (if the weather allows it).