This May I shall visit Stockholm for the first time. Arriving 10pm to Central Station, staying in Östermalmstorg and leaving the following night at 6pm by train. So I was thinking of looking at the view of the City Hall from nearby the Central Station. Then walking and enjoying the blue hour to the airbnb. The following morning walking to Gamla Stan around 8am to 11am. Then Vasa Museum around 12noon. ABBA museum around 2pm. Leaving for the 6pm train at 4pm. I'm mainly visiting friends in the Copenhagen area so Stockholm is just a solo side trip. Thoughts anyone?
Your plan sounds fine except I'd suggest just taking the tunnelbana to your airbnb since you are arriving so late unless your hosts don't have a problem with you checking in so late. I assume you are an ABBA fan. I thought the ABBA Museum was a lot of fun, but be aware they do not take cash.
I think you will need a bit more time for the Vasa Museum. You won't need quite so much time to get to the train station.
Based on my one experience there last August (which may or may not translate to your time in May): The Vasa Museum is most pleasant when it's the least crowded, which is right when it opens. As the day goes on, the ticket line gets long, and the crowds inside get thick. I got there at 9:30 (on a day when it opened at 8:30 - in May it doesn't open until 10:00) and enjoyed a couple of hours relatively unencumbered, but by the time I left just after 12:00, there were so many people huddled around the exhibits, it was hard to see anything.