As the Summer gets closer, the days in Stockholm get longer.  This picture, with Stockholm's City Hall in the background, was taken on May 13 at around quarter to ten at night, and there's still light.

Even though I didn't have the time to use it, if you're going to stay in Stockholm for more than three days between April and October, a great way to tour the city is to use the Stockholm City Bikes program.  To use the system, you first buy either a season card or a 3-day card (depending on how long you're going to stay) and then pick up and return a bike at any of the system's locations, of which there are plenty.  Amazing what you can do when people respect and take care of the public good. For hours of operation, instructions, restrictions and other details you can go to the Stockholm City Bike's website (there's plenty of useful information there).