Sveaborg only has one hostel and an Airbnb (both sold out for the weekend!), but there are plenty of hotels in the city. Many are a short walking distance from the ferry. In any case, the ferry terminal is very close to the city center.
Prices look reasonable for the weekend – I searched Fri-Sun (2 nights) for one person and, as an example, less than 200 euros for two nights at an “Autograph Collection” hotel looks like a steal to me! [This was in January, so might be outdated by now}
The ferry leaves from the South Harbor / Market Square (Kauppatori) in the middle of the image.
