Ekerö is a municipality in Stockholm County on a group of islands in Lake Mälaren. and the only Swedish municipality with two UNESCO World Heritage Sites: Birka and Drottningholm.
As the Ice Age pushed Scandinavian bedrock down, the Stockholm area was submerged below the Baltic Sea until around 1000 BC. Around AD 1000, Birka and Adelsö were important centers of commerce, and the water was part of the Baltic Sea. Stockholm was founded in the 13th century, as the land had risen enough to make Mälaren a lake.
The main island Lovön became royal property in the 16th century, housing the Drottningholm Palace, and royal manors. The towns Ekerö and Stenhamra were villages until the 1970s, and farms and forests dominate the islands.
SL buses from the subway station at Brommaplan.
Strömma runs boats from Stockholm City Hall to Drottningholm and Birka.
Most islands are connected by road. Adelsö is connected to the road network by a free-of-charge ferry. Without steep hills, Ekerö is perfect for bicycles.
Björkö can be reached only by boat, see , and other islands (Kungshatt etc) are reached only by private vessels.
Drottningholm — Although the Royal Palace is situated in the center of the city, the Royal family actually lives at Drottningholm Palace on the Lovö island in Lake Mälaren, 30 minutes from the city centre by public transport. The 18th century palace is beautiful, and much of it is open to the public. The surroundings are well worth a walk as well. Take the green line of the underground (T-bana) to Brommaplan, change to bus 176 or 177 or (mostly during peak hours) 301-336, to Drottningholm. In the summertime, there is also regular boat service from Stadshuskajen (the City Hall Quay) to Drottningholm operated by Strömma Kanalbolaget  (130SEK for a return ticket). Consider the combination return ferry ticket (210SEK, includes the palace and the Chinese Pavilion). But, if you are a student with an ISIC card, don't buy the combo ticket because you won't get the discounts offered by the Palace and Chinese Pavilion. Sadly, there are no interpretative signs in the Palace or in the Chinese Pavilion. So, catch a (free) guided tour, offered nearly every half hour in Swedish and English, and you'll get a lot more out of it. Or, buy a guide book (50SEK). If you use Talk of the town  the mobile audioguide for Stockholm. You can listen at six sights spread over Drottningholm Royal domain in six languages.
Birka — For the real Viking buff, there's Birka , the site of a former city of about 1,000 inhabitants situated on Björkö, an island in Lake Mälaren. The city had its heyday during the Viking Age, from AD 700 to AD 1100. Since most buildings were made of wood, traces of the settlement are hard to spot, and the most prominent "old" structure is a stone cross from the 19th century; a 1,000 year memorial of the advent of Christianity. The small museum (+46 8 56051445, closed during winter) is really only worth the ride if you are genuinely interested in the subject. Parts of the settlement have been reconstructed.
Boats to Björkö are operated by Strömma Kanalbolaget . Birka can also be reached by taking the green line of the underground (T-Bana) to Brommaplan and from there suburban bus 312 to Rastaholm. Once at Rastaholm, walk down the road to your left for 800 m and you'll find yourself at an inn and a jetty. From here, boats depart for Birka at 10:15, 11:00 or 11:30 depending on the season (see timetable ), taking 15-30 minutes to reach Birka.
During the summer, there are several staged events (fairs, archaeology expos etc) at Birka.