One of the best things about river cruising is the slow pace of the trip. Boats float along peacefully and stop regularly at towns and cities for on-the-ground explorations. But if you're short on time, you can see what an 8-day river cruise looks like in less than 2 minutes.
In August I rode on Viking's Romantic Danube river cruise from Nuremberg to Budapest. Here's a view from the bow as the boat sailed from Germany to Austria to Hungary.