Birka (Björko island, west of Stockholm) – this is one of Sweden’s first real towns, and a worthwhile visit for those interested in Viking history (a daytrip must be organized to go there). During Viking times, Birka on the island of Björkö in Lake Mälaren, was the central hub and the most important marketplace in the entire Mälar Valley. It was founded in the 8th century AD, drawing in travelers from other parts of Scandinavia. Nowadays, this site has a wealth of ancient remains that have been excavated and studied since the 19th century.
The Stromma tour company organizes boat trips from the dock at Stockholm’s City Hall (during the Summer) to Birka. A round trip fare through their service is SEK 345 per person. See its website for more details on this exciting trip: https://www.stromma.se/en/Stockholm/The-Archipelago/Day-trips/Birka—The-Viking-City/