Not to pile on, Wray, but here I go piling on!
;)
The Vasa Museum is without a doubt my favorite non-art museum I have ever been to anywhere.
I am not a boat person. I was totally bored by the Kon-Tiki Museum and the Fram Museum in Oslo (which many others rave about). I liked the Viking Ship Museums in Oslo and in Roskilde, Denmark, but neither of them holds a candle to the Vasa Museum. And there are many very fine museums that cover various historical subjects, The Vasa ranks above them all for me.
But, chacun a son gout!
Now, Mary, I think your itinerary as it currently stands is almost worse than the original.
The Royal Armory is in the Gamla Stan. If you go there on your first day, you will be in the Gamla Stan. You will not be able to resist or avoid seeing it! So do it. I think you can easily enjoy the Gamla Stan on your arrival day. And while you're there, if you feel like going into the Royal Armory, or into the Nobel Museum, or the Post Museum, or any other things, do them as time and energy allow. After that, you will have a good idea of whether you want to come back and explore the Gamla Stan more on another day.
Then Day One, you have two museums that are in the Djurgården. Rather than committing yourself to those specific museums and specific amounts of time in each, I suggest you just make Day One your Djurgården day, and do as much there as you want. Whichever museums, whichever outdoor activities (biking, walking, Rosendals Trädgård) appeal to you. The Nordic Museum is right next door to the Vasa Museum on Djurgården, so I don't know why you have moved it to a separate day.
I should have given you a disclaimer right from the start, though. I'm not a careful planner, and I don't really think making careful plans is a good way to discover the joys of a city like Stockholm. I just woke up each morning I was there and over breakfast I planned out the day. On the spur of the moment one day I decided to take the train up to Uppsala. I also took a lovely ferry ride to the delightful little island of Fjäderholmarna at the suggestion of a very friendly local woman I met on the train. I'm not suggesting you should do any of these things; only that you should allow yourself the freedom to explore as the spirit moves you.
One place I didn't get to and wish I had, not mentioned by anyone on this thread so far, is Millesgården. (I was going to go the day I decided to go to Uppsala. No regrets -- Uppsala was great -- but a reason to go back!