There is only a handful of WH sites associated with the Vikings, and the major reason for this seems to be that they just didn't leave that many traces. Their buildings were made of wood, so the major Viking remains are runestones and burial mounds. These are also what you can see in this site, spread over 2 islands in Lake Mälar, just outside Stockholm. Birka, on the island of Björkö, is only reachable by boat, and there is a daily excursion boat leaving from Stadshuset. The island includes a decent museum with many findings (although most are in Stockholm museums), and the guided tours take you to the major burial mounds and modern additions (a chapel and a large cross dedicated to St. Ansgar, who - unsuccessfully - tried to bring Christianity to Birka). Birka was a large trading post in its heyday, and one of only a handful of Viking-age cities. The king, however, resided in Hovgarden, on the island of Adelsö, just a short boat ride away. I wasn't really sure how to include this in my travel plans, but in the end, it was pretty easy. Just 30 minutes after the boat to Stockholm left, another boat made the trip to Adelsö, and I could explore the site on my own. There are burial mounds there as well, plus a few scattered runestones, which I didn't see in Birka. So I think both islands are worth a visit, even if the bus trip from Adelsö back to Stockholm took over an hour. This is one of the few insights into authentic Viking life that one can get anywhere - and there is a realistic imitation of a Viking settlement close to the museum in Birka.