This September my husband will be in Sweden for the first time. We have four unplanned days that we would appreciate some feedback on. We will have been in Copenhagen, and will be headed to Stockholm. We plan to pick up a car in Malmo and drive up the west coast. Our tentative plan is to stop in Ystad (one night), Karlsrona (one night), and two nights in Kalmar, which gives us a day to explore the island of Oland. On the fifth day we will drop off the car, and take the train to Stockholm. Any feedback, suggestions, would be appreciated.
I assume you mean the south coast, not the west coast.
I can't comment about Ystad or Karlsrona, but I did spend a couple of nights in Kalmar and spent a day exploring Oland. I stopped at the TI just across the bridge, and they were enormously helpful. They asked me about what I enjoy seeing and then helped me plan a route. Note that the island is bigger than you may realize, and I ended up exploring only the southern half. I made it down to the very southern tip, where there's a lighthouse and a bird sanctuary. For me the highlight of my visit to the island was Borghom Castle. Rick is kind of ambivalent about it in his guidebook, but I really enjoyed exploring it. I stopped there first thing and enjoyed the shadows created by the morning sun.
I also visited Solliden, the summer home of the royal family. The gardens are lovely, but there are lots of other lovelier gardens in the world, so I'm not sure it was worth the time I spent there.
Rick seems to like Eketorp fortress, but to me it felt like a bit of a tourist trap. It's virtually completely rebuilt, so there's nothing authentic about it, and there are performers in period dress. If you're into that kind of thing, it's worth visiting I guess.
I also really liked seeing Ismantorp. This is a ruin that has not been restored, so it felt more authentic to me than Eketorp.
As I said, your best bet is to stop at the TI and get recommendations from them that match your interests.
By the way, I also really enjoyed Kalmar. My pics from Kalmar and Oland are at https://goo.gl/photos/VpRDHU5CaPhNzZtS8, if you're interested.