From my years as a guide I have built up a lot of experience in putting together good excursions. In addition to the standard excursion, I can arrange excursions that are designed especially to cover your area of interest. Write!

Here are a few examples of standard excurions:

The best of Stockholm in 3 hours
Tour by bus/car and walking

This is the most optimal standard excurions considering both time and coverage.  
The tour starts with a tour by bus or car, depending on the number of people.
Stockholm is built on 14 islands, which is why the use of some kind of transport is recommended.
During the excurion you will visit two squares, one is situated on Riddarholmen which is situated in lake Mälaren, and one on Stockholm´s largest island - Söder. The square on Söder is situated 27,8 meters higher than the Baltic Sea. 

The excursion continues through the central parts of the modern Stockholm -  Stockholm City. The last part of the excursion is a walking tour through the Old Town, where you will pass narrow streets and alleys and experience Midieval Stockholm.

Midieval Stockholm
A two-hour walking tour

You will visit the three of the 14 Stockholm islands that make up Midieval Stockholm: Stadsholmen, Riddarholmen, and Helgeandsholmen.  
On Helgeandsholmen, "The island of the Holy Spirit", you will get to now how the island got its name and also about modern life on the island.
 
On Riddarholmen you will get to know about the Franciskaner monks who build their monastry here. During the summer time, you can visit the church on Riddarholmen. Here Swedish kings have been buried since the 13th century. Here is also a square with an excellent view over western Stockholm including the City Hall. In the City Hall, the Nobel banquet is held each year and it will probably be interesting for you to hear more about the Nobel prize. The tour ends up in the Old Town and you will be told about its history, the history of the Royal Castle and the history of the Swedish Monarchy.