Following the ancestors to Sigtuna
The Vikings left Birka suddenly in the late 900s;  scholars think they decamped to Sigtuna, the oldest Viking city still in use in Sweden, founded in 980. 
Sigtuna's the place to see rune stones, named for the alphabetic letters, runes,  used by the Vikings to encode their Norse language.  Many of the stones are quite touching monuments erected by the living to honor their dead relatives.  Our favorite inscription: "Anund had this stone erected in memory of himself."  Bold, Anund. 
Christianity found a bit more traction in Sigtuna than in Birka.  There are many ruined, and many functional, churches here.
Hugh took this artful shot of a familiar Swedish icon, a figure carved by a Viking from an elk horn, now in the Sigtuna city museum.
The Sigtuna city museum also has lots of reminders of how far the Vikings ventured.  This is one of the painted Russian eggs in their collection.