We have reserved our shore excursions for the cruise portion of our trip through the cruise line. While we could explore the ports on our own, our experience has been that the shore excursions we've done through the cruise lines (both on Disney and Royal Caribbean) have met our family's interests and needs. The most important need we have is to be assured that the ship won't leave us behind if we're late back from an excursion, something that our older DS has worried about since our first cruise five years ago, and we have been grateful for that when watching fellow passengers running for the last tender and wondering if they would have to jump from the pier to the retracting gangway when they were late.
Our first shore excursion will be in Oslo. The highlights of the tour include driving to (1) Bydgoy Peninsula where we'll tour the (2) Viking Ship Museum, (3) the Kon-Tiki Museum, and the (4) Norwegian Maritime Museum. Kon-Tiki is a balsa-log raft that Norwegian scientist Thor Heyedahl sailed from Peru to Polynesia in 1947. (I know that only because I read it on www.Frommers.com.) If you've not used Frommer's for either a guide book or just online reading about your destinations, it's an easy to use site chockful of information.