We loved our Viking River Cruise. We did the 7 night Rhine Getaway in November of 2013. My wife and I are in our 30's. Most passengers were retirees, but there were two other couples our age and one teenager.
Since you're new to cruising - just know river cruises you're not going to get seasick. There is no assigned seating when dining and everyone was so nice. At night you eat and sleep, during the day you're taking tours and walking around towns. Not sure if you drink or not, but having unlimited free alcohol was great. They serve wine and beer from the region and will always have a house red or white.
In November, Europe was real cold but we were prepared (long john's, heavy jackets, layered well). I'm assuming it's still cold in April there.
I can't wait to do another cruise with them. We had done it for our 10 year anniversary trip. If it wasn't for work or kids, I'd take many more cruises. Hopefully in the next year or two we'll do the Danube.
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