I'd say the Vasa Museum is very nearly a must. I am not interested in ships or in that historical era, so it's far from my all-time favorite museum, but it is brilliantly done.
Gamla Stan, the medieval old town, has some intensely touristy streets, but others are comparatively quiet. That's a good area for walking around and appreciating that you're far from home; just make sure you get off the beaten path for part of the time. If you're going to eat there, you should at least Google and check ratings. I imagine there must be some poor choices in that area as well as good ones.
I stopped several times at CitySallad, Skeppsbron 6, near the NE corner of Gamla Stan. It serves the neighborhood office workers, so it's only open Mon-Fri 7:30 AM to 2 PM. There are about 10 salads available in a cold case. The last time I was there, there was no one in the front of the shop, so I just rang up my own order (all salads are the same price) and paid by credit card. Salads come with a cold drink and some form of bread. I don't remember whether you need to grab your own dressing or it's packaged with the salad. The cost is now 130 SEK (under $15 US). CitySallad menu You'll need to right-click for an English translation.
Stockholm is a city with beautiful architecture in a beautiful setting, so it almost doesn't matter what you go see as long as you keep your eyes open along the way. I'm an art lover, so I spent a lot of time in the art museums. I especially liked Millesgarden, but it's well outside the center, requiring Metro + bus + walking, so a potential visitor needs to weigh the trade-offs before planning a visit. Stockholm has a wide variety of museums and sights; your crew might be interested in other things. Pick up a guidebook with good coverage of the city so you have a decent idea of what is on offer. In the meantime, Wikipedia has this list of some of the most prominent museums in the Stockholm: https://en.wikipedia.org/wiki/ListofmuseumsinStockholm
A day trip to Uppsala is a nice option.
