Stockholm is a city that enchants with its historic buildings, and narrow cobblestone streets. Everywhere there are bridges spanning the waterways that divide the city. Spread over multiple islands, hop on hop off water ferries are a great way to explore.
The historic center is called Gamla Stan. No visit here would be complete without a stop at the Vasa Museum. Housed in this museum is an original warship from 1628. Salvaged in 1961, this historic ship actually sunk on her maiden voyage due to design flaws. Visitors can get an up close look at this one of a kind piece of history, and learn more about that era. Tours are available in English.
The changing of the guard is another enjoyable pastime here. Watch as the horses get so close you can almost reach out and touch them.
For kids, there is a large amusement park called Grona Lund, with lots of rides and shows.
If you are arriving by cruise here is the wikiport information.
Cruise passengers need to know there are two HO HO ferries that depart from the cruise pier. Of those, Stromma has the easier to navigate site in English. The other Ressel shows a schedule(circle line pdf) for the other ferry, but not much else in English.
The most important thing to keep in mind is the ferries go in different directions, so you must plan how you will visit the different stops. Make sure that the ferry you choose, goes in the direction you need, to return to the cruise pier before the ship departs.
Here is one last helpful post about the ferry boats:
No matter what you choose to do, there is lots of sites to see in Stockholm.