Sigtuna is a small town situated north of Stockholm and can be reached by taking a bus from Märsta.  There are a couple of shops for souvenir shopping and of course the famous St Olaf's Ruins. Take a short walk down to the pier and catch some wild ducks in action
One thing I really liked about Sweden is the ease of getting around with Google Maps.

One of the oldest church built and probably the first in Sweden, St Olaf's Ruins originally built in the year 1100 but the construction to extend the church did not complete after the as the archbishop has decided to move to Gamla Uppsala 1200. All you see here is the ruins of the church.