We are planning to train between Stockholm and Copenhagen and could spend a night between the 2 cities. Any places that would have things to see and do?
There are several good options. Any particular kind of place you are looking for?
Historic or scenic maybe. I saw there were glass factories but we will be carless.
The glass factories are difficult-to-impossible to reach without a car. Believe me, I was willing to jump through some hoops to make it happen, but it just didn't seem practical. Sometimes you just have to give up on something like that, which is what I decided to do. However, the Smaland Museum in Vaxjo had good glass exhibitions in July. At least part of the displays were temporary exhibitions, so I don't know what will be displayed at the time you might be able to visit. Vaxjo is accessible by rail, and I was glad I included it on my itinerary.
We travelled from Stockholm to Copenhagen via Gothenburg in September 2022. Gothenburg ("Yotty-burough") is a lovely small city with a beautiful harbour and glorious city park. We also enjoyed the Volvo museum which is one of the bette car museums I have been to - so well curated that non-car enthusiasts could still enjoy it.
There are many glass factories in the area and it can be tricky to visit them without a car. It will not help you at the moment, but hopefully in a few years the one in Skruv will be easy to visit. A small family run place. https://skrufsglasbruk.se They have also been awarded the title Purveyor to the Royal Court of Sweden so they are obviously not too bad. It is just a few hundred metres from the former railway station, and the plan is that the trains will stop there again soon. Until then, the official website of the area is https://glasriket.se and you can contact them and see if they have any suggestions. They probably know the buses in the area better than I do.
Kalmar can be a place to stop. Based on the attention it gets on this website I'd say it's overrated, but it certainly is worth a night. A suggestion: When taking the train from Kalmar to Stockholm (or v.v.), the system will usually suggest a route with a change in Alvesta, as it is the fastest option. But the route with a change in Linköping is a more scenic and nicer one in my opinion. And only about 20-30 minutes slower. To show that as an option, add Vimmerby as a via-station.
For a smaller town rougly halfway, consider Eksjö, in the northern parts of Småland. A charming small town with a well preserved wooden town centre. Pretty easy to reach as well, only 10 minutes by local train from Nässjö, where the Copenhagen-Stockholm trains stop.