Can't help much with activities, but I've stayed in Gegenbach and Freiburg. Both are lovely. Gegenbach is a village, and Freiburg is a beautiful town. We also nipped over the border to Strasbourg in France, and visited Baden-Baden, both worth visiting.
I went in November for a weekend away and we drove, but were only coming from the southern Netherlands.
The world's biggest cuckoo clock is hilarious and not to be taken seriously.
www.youtube.com/watch?v=R4ca99hHB38 
There's walking in the summer, ski-ing in the winter, and the flammekuechen are delicious. (a bit like a pizza but sour cream instead of tomato sauce).
There are definitely some MNers living in the area - I've seen them in the Living Overseas topic, so it might be worth asking there.
It is possible to drive it in a day from UK, but I'd be more tempted to break the journey in France if travelling with children. And don't expect German motorways to be wide 4-lane affairs. They might have no speed limit but many of them are more like 2-lane A roads.
I like Germany. The food is delicious, and there's plenty of it. am less keen on the wine though