A 1.5 hour trip in a traditional sailing ship that picks you up from the touristic harbor in the city center, right by the City Hall (same pier as the boat leaving for Bigdøy,) and slowly takes you to see the interesting Opera house, you go through the main islands close to the city harbor and then the last stop is at Bigdøy, the area hosting famous Oslo museums such as the Vikingship museum, Fram Museum, Folk Museum or the Maritime Museum. You can obviously step off (or on) at any of these locations if you desire.
But I personally recommend a full trip on the boat since it’s a rather relaxing experience to sit on the deck (mainly with good friends around) and simply enjoy the view and the fresh breeze. And if you’re lucky, in the warm sun as well. Another bonus offered by such a cruise is that it allows you to view the objectives mentioned above, and also the Akershus Castle, from totally different angles than what the shore offers.
It was also a lot of fun to wave at people who were either getting fried under the warm sun on the islands or spending some time on the deck of those huge cruise boats that we passed by.
The company seems to offer a variety of other types of cruises in the fjord and I can only think they’re worth trying so one can grasp Oslo’s beauty as seen from the water.