If you are trying to save money, book the lowest cabin level and just use the included excursions. You will have a great time.
If you have more money available there are many ways to spend it: Shopping on the cruise, extensions, excursions, better rooms, better cabin class, a higher level of flights.
On a Viking River cruise I would upgrade the cabin last. Everything else will improve my enjoyment more than a better cabin. Extra excursions where I have a special interest would be first on the list of upgrades.
Not quite on topic, but on Viking Ocean upgrading the cabin is a higher priority for me. It gives you a more specialty restaurant reservations and a better chance of getting optional excursions you want (since you book earlier). But DV and booking the minute excursions are available is a bigger improvement than any of the other classes.
For flights, Premium Economy or Business give you less hassles at the airport and you can sleep better -- which can help you enjoy the start of the cruise.