Taking the overnight ferry to Helsinki is one of the easiest (and most cost effective) trips you can make from Stockholm!
You can book a cabin on one of the cruise ships that make daily crossings to the capital of Finland. The cruise consists of two nights on the boat and a day to visit Helsinki in between.
Helsinki is almost impossible to see in one day, but you can give it a good try before hopping back on the ship for your return back to Stockholm! (You would have approximately 6 hours in Helsinki before your ship departs again.) We recommend catching the number 4 streetcar that will give you a nice trip around the central parts of the city in less than an hour!
An overnight Ferry ride (17 hours)