I visited Hovgaden on Adelso Island by #312 bus from Brommaplan T-bana station. There is a daily ferry from Hovgaden to Birka, but as its departure is not co-ordinated with any bus arrival I didn't go there. From other's reports, I didn't miss much. I did see the island in the distance from the Hovgaden fortress site, and found some inscribed runic stones near the Adelso church.