Norway + which country?

Around 15 years sgo, we did a house exchange in Copenhagen; and just loved Denmark. There are plenty of castles in Denmark. We enjoyed visiting them, as well as several of the open air museums and living history sites. The architecture is different from that of Europe further to the south, so many churches and other historic buildings were of interest. The Viking Ship Museum in Roskilde and the cathedral there are notable, and The National Museum in Copenhagen is not to be missed.

I would guess that despite his inclination to seasickness, your husband could tolerate a boat tour around Copenhagen. It’s a great way to get an overview of the city, especially the contrast between the ultramodern, like the Opera House, and the old and traditional sections.
Copenhagen is also great for strolling. In the summer, people are out ‘til late, just enjoying the long hours of daylight.
Sweden, on the other hand, was, to me, just o.k., not nearly so appealing.

With a maximum of ten days (how many nights is that?), I'd stick to Norway. You've got two significant cities (Oslo and Bergen) plus the fjords. Most people can see some of fjord country in a (long) day by following some version of the Norway in a Nutshell route, but since boats are out for you, you're going to need to put something together on land. I have ignored the driving advice I've encountered since I don't rent cars in Europe, so I don't know how much time you'd need to get to nice fjord views by car. Bus would be more of a challenge because service seems very infrequent in rural areas, and I wonder whether your husband might have issues on the rural buses. I have definitely felt queasy on European buses traversing curvy roads.

There are some earlier discussions about driving in fjord country on the forum--probably all in 2019 or earlier. I'd recommend scanning thread titles, working backward, to see if you can spot those that might cover your needs.

Have you taken a look at a guidebook to Norway to see where you'd want to go? Even if you manage to squeeze your Norwegian must-do's into, say, 6 full days, that doesn't leave you much time to travel to another country and see it. The most I think you could reasonably hope for would be seeing the capital city. In that case it comes down to whether you'd rather go to Stockholm or Copenhagen.

Edited to add: Here's a current thread that delves a bit into seeing the fjords by car:
https://community.ricksteves.com/travel-forum/norway/norway-fjords-in-7-days

Considering combining Denmark with your Norway trip. CPH is SAS' hub. You can do a multi-city air, stopping for serval days to visit Copenhagen before continuing to Norway.

Our big plan for last year, canceled due to the pandemic, was to actually start in Scotland, then head to Denmark, and finish in Norway. Whichever order you do it, Denmark would pair well with Norway.

You’ll do it before we do - we’re now going on a safari trip to Africa in April 2022, so Norway planning’s still sometime in the future.

For 10 days it might make sense to just focus on Norway. That will give you an option to really see the fjords. On the other hand, Sweden and Denmark are pretty close so it is easy to add a few days there. Castles are rare in Norway but there are many in both Sweden and Denmark, so if you want to see castles, just going to Norway will not do. Anything else than castles you want to see?

One option could be to start with a few days in Copenhagen, take the train to Gothenburg for a night there before continuing to Norway. Or maybe start in Stockholm and make a stop in e.g. Karlstad or Gothenburg on the way to Norway.

Stay in Norway all ten days. And add more time to your trip if possible.

We have been to all three countries. All three are great to visit.
However, I would put Norway at the top, by far. The amazing scenery is awesome.
Consider doing a cruise up the coast of Norway to the North Cape. We did one from Copenhagen.

Stockholm and Norway Aritic Circle
https://www.cruisecritic.com/memberreviews/memberreview.cfm?EntryID=659526&et_cid=3214772&et_rid=17221689&et_referrer=Boards

Norway is deceptively large, so I would maximize time there, even though daily costs are high.
If you have to transit through Copenhagen or Stockholm, then perhaps you could spend 2-3 nights in either place, but that is not what I would do.

I can only second those voices that recommend focussing on Norway. Even then, you will only be able to see a relatively small part of the country; Norway is indeed surprisingly large.

The fjords are spectacular to see from a boat, and I think even your husband should be able to cope with that boat ride. There'd have to be a storm for serious waves to develop on those fjords, and you wouldn't go in a storm anyway.

For castles in Norway, there is actually a list on Wikipedia, so pick and choose.

Historic sites: Check out the stave churches. We also really liked the Edvard Grieg Museum Troldhaugen near Bergen. The visit included a half-hour lunchtime concert.

Landscape lovers and photographers go crazy about a totally different part of the country: the Lofoten up in the north. I haven't been there, so I can't speak to that from my own experience.

On a side note re seasickness:

Since I suffer from seasickness too, I have recently looked into something I had heard about: glasses that have a built-in water level, fooling the eyes into seeing that as a stable horizon. They are the ugliest, most ridiculous-looking glasses you have ever seen in your whole life. But since they aren't that expensive, I am seriously considering getting a pair and giving them a try. Can't be worse than spending a wonderful boat trip with a paper bag in front of my face! Maybe I'll even get two, so I can find someone else to wear one and make a fool out of themselves with me. :-)