Have you looked at barges? That is another, slow paced way to enjoy the European countryside. On a barge, the teens could get off, jump on bikes and ride up to the next town (or further) and meet the barge. That would give them some adventure and a chance to get away from the "old folks". Don't know if they allow that on the river cruises though.
I've known some folks that have done a barge trip and loved it. My only experience with river cruising was the Nile River and that was incredible! I will be doing the Yangtze in 2008.