I have asked the very same question about Belgium. I really liked the answers I got from Must see places while going through Belgium
Edit: I will be traveling by car from Baltics to Netherlands or Belgium with a group of 6 people, ages varies from 20 - 50. Our direction really depends on the weather and we'll start working our way back from there. One way or another we will be interested in the middle part of Germany (part between Hannover and Nürnberg).
We don't have anything special were interested in, we would just like to see as much as possible while driving back to the Baltics. Also we would like to avoid tourist traps where one could possibly spend a whole day, like amusement parks, big cities, bars & pubs etc.
We're looking sights where we could basically drive through (spend an hour or two and then move on). We're interested in anything that makes average Joe from another country go "wow" one way or another.
Earlier years we've used technique where we've looked POI's 50-100 km along the route from GPS and going through the country in a flowing manner.
Some style examples we've liked: Hill of crosses in Lithuania, Wieliczka salt mine, windmills in Netherlands, some great Castles, Nature beauty
What are a must-see places in Germany (middle section)?