Clearly a Must-Do. Started in Stockholm you get an appox. 2h Boad ride through Mälaren with some Informations at interesting Points. On Birka awaits you a replica of a small Viking Village and a interesting Museum. But the most interesting part are the guided Tours around the Island. At the time I was visiting, there was a Tour for the "Viking Halloween" called Álfablót. It was only in Swedish but there was a Transcript available into English. Then there was a guided tour in English, witch was very good and interesting. The guide was very sympathetic. You get a good view onto the Island with his graveyards, where the city was located and the history about the Island and the arciological work. At least you can say, that around 3,5h is maybe a bit to short for one or the other, but it ist absolutely with it.