Silja Line operates direct services between Stockholm and Turku, via the Aland Islands. Services direct to Helsinki are also available, although these tend to be more expensive and leave early in the morning, meaning that you would need to stay in a hotel overnight to take one.
Generally, there are two services per day in either direction on this route, allowing you to leave early in the morning for a day sailing, or later in the evening for an overnight trip.
Cabins are available for either option, although it is obviously advisable to book a cabin for the night journey.

