One of the best places in Sweden if you are interested in vikings and who they were, but only accessible in summer during the archaeological digs unless you have your own boat. The old viking settlement at Birka is very alive since archaeological diggings still take place every year during the summer months with new finds. The people who work with those are your guides on tours around the islands (in Swedish, English and sometimes German) with various themes depending on year. The price for the tour is included in the ticket. Most tours pass the fields with viking burial mounds and it feels a bit like walking in an outdoor church if you see what I mean. You will also see the huge cross dedicated to Ansgar, known for spreading Christianity in this region of what has since become Sweden.
You get there with a boat from the city centre (a scenic and guided 1 1/2 hour trip) and once there, you will find a museum with various exhibitions and a shop (starting point for tours), as well as a cafeteria by the jetty if you haven't brought your own picnic food. If the weather is nice you can sunbathe on the tiny beach too. Just remember what time the boat leaves. See my Ekerö page for more information on this part of Stockholm. The museum also has information on the bigger, neigbouring island of Adelsö where the kings of Birka lived at Hovgården (also a part of the World Heritage even if only the foundations are left) to overlook their subjects on the smaller island opposite. For a few summer weeks there is a small boat between Adelsö and Birka too.
The Swedish National Heritage Board has leased the guided tours and transport out, and this is currently run by Strömma, the company running sightseeing in Stockholm too. Therefore you will find further practical information on their homepage too: http://www.stromma.se/sv/Skargard/Stromma-Kanalbolaget/
Birka is a former Viking town that has been excavated and studied by archaeologists. A two hour boat ride from Stockholm, it is sold as a day out and a chance to learn a lot about vikings.
Birka is a pretty island, has some nice trails, swimming, and offers great views.
However, for those wanting actual viking info it's a steep price and a long visit (you have to spend at least 4 hours on the island) for what littel you get.
The museum is tiny, the little viking area with actors is interesting but the activities (bow and arrows, iron working, plus one or two others) are more for kids and cost money. The trails are nice but beyond the ruins of a wall, the burial mounds and a modern monument there's littel viking stuff there.
It's pretty skimpy and not worht the 255 SEK.
BIRKA was a world heratage site since 1993. Its located on the island of Björkö in Lake Mälaren.
This island illustrates the trading networks of the Viking Age and their influence on the history of Scandinavia.
Here you can see:
-Great landscapes with grazing animals and verdant fields
-The Birka Exhibition : models, archaeological finds and reconstructions
-Thor, Christ and Allah / the People and Gods of Birka. Here you can see oriental coins and remnants of clothing from the Far East, which where found on the island
-The Museum Shop / souvenir shop
Open to the public from May through September
Take a trip out to Björkö/Birka, and see the first Swedish Town. Only possible end-April through end-September).
Site of the first Swedish capital, World Heritage Site. Great tours around the island by archeologists. A wonderful spot for a summer picnic (but watch out for the ticks, they're a lot of them and mostly all of them carry lime disease - Get your shots). See the 'World Trade Centre' travelouge.