Old Town or Gamla Stan is in the center of Stockholm and is the oldest part of the town. It has shops, cafes, wine bars, cocktails, antiques and beautiful architecture--it's worth at least a short walk during your trip.
The Vasa is one of the most visited museums in Scandinavia--home of the ship Vasa, a preserved 17th century ship that capsized and sank in 1628.
If you have free time in Stockholm we highly recommend checking it out.
Stockholm's City Hall is a worthwhile tour if you can fit it into your schedule.
It is the venue of the Nobel Prize banquet and has amazing views of the city.
They have guided tours in English throughout the day.