Solo traveler, mid 60s; I am stopping in Stockholm for 4 days after a week in Abisko/Kiruna.
What are options for going out at night for a solo traveler? Nothing crazy but sitting in the hotel seems a waste.
When? What are you looking for? In which part of the city are you staying?
In March, staying in Gamla Stan
If you like opera, you can see if anything is of interest at the Royal Opera (Kungliga Operan)
https://www.operan.se/en/calendar/
It is a beautiful opera house.
The Fotografiska Museum is open until 11pm.
The Royal College of Music often has concerts in the evening
https://www.kmh.se/in-english/concerts--events.html
You can go to the cinema. If you choose an English language film you should be fine. Films are almost always shown in the original language with Swedish subtitles ( the exception can be some showings of films for very young children).
You could see a play at The Royal Dramatic Theatre (Dramaten). Often the plays are subtitled in English. Their English website has this info.
https://www.dramaten.se/en
I enjoyed dinner and the evening here:
Aifur - "Welcome to the world’s best .. and only Viking Tavern."
https://aifur.se/en-GB/aifur-en/
I came across this in my research.
Stockholm Ghost Walk https://www.stockholmghostwalk.com/en/
In March there will still not be that many outdoor activities available. But just walking around Gamla stan and Riddarholmen is not a bad idea, they are very charming in the evening, especially since they are not that crowded. Or just take the ferry to Djurgården and back for the evening views of Stockholm.
Checking if there are any interesting concerts available is also not a bad idea, there might be some nice options depending on what kind of music you prefer.