The area prospered during the Viking Age, leaving a heritage of runestones, graves and monuments, while Lake Mälaren was part of the Baltic Sea. Sigtuna was founded just before AD 1000, as Christianity arrived, and Sweden was united as a single kingdom. With a royal mint and a bishop, Sigtuna was effectively the capital of Sweden until power shifted towards Stockholm and Uppsala in the 13th century. After three fires in the 17th century, Sigtuna was a ghost town for 250 years, revived in the 1910s as a nationalist project.
The Arlanda airport opened in 1960, and has expanded step by step. Märsta, at the railroad between Stockholm and Uppsala, grew side by side with the airport.
Sigtuna is part of Stockholm's public transport network. 
If you touch down at Arlanda, and you want a chronological approach to Sweden, you could rent a car at the airport (or take the public-transport bus) and start exploring Sigtuna, going on to Uppsala or Stockholm.
Hiking, cycling and bathing during summer. Ice-skating and Nordic skiing during winter.