Go over to the River Cruise forums on cruisecritic.com. Lots of reviews of the various lines and very active board.
I have been on several and we are very active. It is a good fit for us when we travel with my mother. AMA Waterways is our favorite for the more active style and younger demographic - their newest boat even has a swim up bar in the pool!
They offer fast walker tours (we normally skip anything organized and DIY) and extended bike rides (18km or so each day) which are a great way to spend a day and see the countryside.
We like their theme cruises (wine and Christmas markets) as these also attract a younger and more active crowd.
Stay away from Grand Circle as they are AARP only.
Look for a 7 night itinerary with extended times in port and overnight stays.
We do pre and post time in Europe on our own.
While independent land travel is our preference, not all river cruise lines are created equal and with some research you will be able to find a line and itinerary to fit your needs.
The longer cruises attract older passengers. On a wine cruise last year at least half of the guests were 40-50 something and since the events are co-sponsored by wineries, they had their management team (30s) aboard, along with wine club members. Some of the added features included winery tours and tastings and were a lot of fun.
You can PM for more detailed info if you wish.
Edited: 02 October 2012, 18:24