Östhammar is a small town on the northern Roslagen coast, east of Uppsala and north of Stockholm. It is quiet for much of the year, then becomes more obviously coastal in summer, when boats, beaches, and nearby Öregrund pull Stockholm and Uppsala visitors toward the Baltic.
The town itself is only part of the draw. The wider area has three good threads: archipelago life around Öregrund and Gräsö, old Walloon ironworking at Österbybruk and Forsmark, and the strange modern presence of one of Sweden's nuclear power plants beside an older industrial estate.
Östhammar is not a first-time Sweden essential, and it should not pretend to be. But if you are already in Uppsala or Roslagen with a spare day, it gives you a compact version of the region: red wooden harbour towns, ferries, forested islands, manor-like ironworks villages, and a coast that still feels lived-in rather than staged.