I agree with Heather about Oslo. It is completely doable on your own with public transportation and the Olso card. However, we decided to hire a guide to spend the day with us just for the insight into the area and culture. We used
http://www.guideservice.no/en/. We still used public transport and the Olso card, we just had a wonderful guide explaining everything as we went along. Was it absolutely necessary? No, but we really enjoyed the added information and our guide was a pleasure to spend the day with. If we go back to Oslo again, we'd probably just go it on our own as we have a feel for the lay of the land now.
As for Stavenger and Kristiansand, these were the only ports that we used
DCL excursions. Our time in port for Stavenger was very short and we arrived early (around 7 am) and were scheduled to leave around 12:30 or 1:00. We really wanted to see the Lysefjord (highly recommend!!) and the only real option to do that was through DCL. I couldn't find any other options independently that left that early that didn't required renting out an entire boat - for major $$$$$$$. I wish we had more time to explore the town. The short stroll we took was delightful. The ship docked right in the heart of town, so it is an easy walk.
As for Kristiansand, I honestly couldn't figure out what the heck to do there, so we took a DCL excursion to a small fishing village (Hollen) and an open air museum. It was a pleasant few hours, but nothing too exciting. I wish that we could have skipped Kristiansand all together and either had more time in Stavenger or possibly gone to another port further north into the fjords.
And while this isn't about Norway, I have to second Heather's comment about Kirkwall. For anyone that will be visiting there next summer who wants an independent guide, book today. I'm serious. You may already be too late. We used Orkney Aspects and it was wonderful. We had Pat Stone, the owner, as our private guide and she told us that her company was completely booked for two out of the three times the Magic will be in port in 2016 already. She also told us that there simply aren't enough buses on the island to accommodate a ship the size of the Magic for full day tours. That is why most of the DCL excursions were only 3 to 4 hours, which is a shame. It's a fascinating place and even a full day only scratches the surface. One tidbit we found interesting was that one of her guides had a group of Disney imagineers on the day we were there. Even Disney employees book independently!