If you want to explore then I would say you would need a car. We went to Bergen Belsen which was a POW camp and then a concentration camp during WW2. Some of the photos were horrific so I had to steer my 9 year old away from them. The town of Celle is lovely as is Bremen. My daughter got quite a few bites I think from playing in the sand at the park so remember good insect repellent!
The Black Forest region, and Alsace are a must see. There were some issues with traffic congestion around Freiburg and Emmendingen which was on the local news. A Rhine cruise from Breisach is a treat, and Europapark great for the kids. The Alsace wine route along the Selestat, Colmar, Mulhouse motorway is lovely. See Lake Titisee and the largest cuckoo clock in the world. Fit in a trip to Basel in Switzerland. Lots of bird watching for nature lovers. Plenty opportunities for cycling and walking.
People very helpful and friendly. The whole area was spotless and everything is so well signposted and the roads well marked to help with the driving on the right side. The Black Forest area as a whole was massive and equally beautiful. So many outdoors activities to do and as I mentioned the Schwartzwald card is a worthy investment. Europa Park was a really good theme park too. Would recommend car hire to reach the many attractions. If you are looking to lounge around in a bikini and catch the sun all day then this would not be the holiday for you as the weather, although pleasant, was not scorching. If you want to zip wire, canoe, toboggan, cycle etc then this is for you.