We took two independent excursions, one ship excursion and one day on our own. I would say the independent excursions and ship excursions are probably more similar than in other countries / destinations. The golden circle tour, as an example, (we did SAGA tours) - is basically 3 stops and you are on your own for 1 to 2 hours when you are at the stop. So basically it is transportation, however, I enjoyed the commentary on the way there. Personally, I do not like to drive on vacation, since I do a lot of driving in my normal life, so I like excursions - others feel differently.
I can tell you we had two overnights there (on the ship) and the second day in each port was a short day (ship departing at 2 pm). So I booked a ship's excursion on those short days since the ship leaves mid day. I am glad I did. Our independent excursion on one of the full days (first day in port) was late back to the ship by 1.5 hours, but it was not a problem since we had an overnight then. This was not the excursion's fault, but there were traffic issues. I never would have expected a traffic jam in Iceland. There is not that many vehicles, but all of the roads seem to be 2 lane and one accident or issue can back things up.
Although we have not been on Viking ocean ships we have been on their river boats. I would expect Viking to have nice excursions planned. I liked Viking a lot, except for the brochures they mail you every week for the rest of your life!
I hope you enjoy your trip, we really enjoyed Iceland.