One of the best ways to travel to and from Finland is by sea.
The boats to Estonia and Sweden, in particular, are giant, multi-story floating palaces with department stores.
Both Silja and Viking offer overnight cruises from Helsinki and overnight as well as daytime cruises from Turku to Stockholm, usually stopping in the Åland islands along the way.
These are some of the largest and most luxurious passenger ferries in the world, with as many as 14 floors and a whole slew of restaurants, bars, discos, pool and spa facilities, etc. The cheaper cabin classes below the car decks are rather spartan, but the higher sea view cabins can be very nice indeed.
Helsinki and Tallinn are only 80km apart. Eckerö Viking Line and Tallink Silja operate full-service car ferries all year round. Depending on the ferry type travel times are from slightly over two hours (Viking Line and Tallink Silja's Star, Superstar and Superfasts) to three and a half hours (Eckerö and Tallink Silja's biggest cruise ships).
Note that many locals use the international ferries for overnight 'booze cruises' and they can be noisy and crowded at weekends. There are also ferry services to Finland from Rostockand Travemünde (Germany). Several major international cruise lines call at Finnish ports.