Even if you only spend a few days in the "Venice of the North", I would still advise you to get on a boat and go out to the beautiful archipelago surrounding Stockholm.
The closer islands in the archipelago, such as Waxholm, Finnhamn and Fjäderholmarna, are only an hour or two away and are nice places to wander around for a couple of hours.
It's a lovely way to spend a nice day or half-day out, and if you go during summer, the archipelago is a great place for a summer picnic and swimming - that's what the locals do.
You can catch boats from the ferry terminals at Slussen and Strömkajen, below the Grand Hotell. www.stockholmtown.com/templates/substartpage____2409.aspx?epslanguage=EN; www.waxholmsbolaget.se/
The ferries crossing the sea between Stockholm and Helsinki are notorius for on-board alcohol-induced shenanigans. It takes about 12 hours from Stockholm to Finland and most passengers opt for a Nordic buffet with a disco and a lot of booze afterwards. The ferry terminals are very close to Helsinki city centre, which makes the ferries convenient for a day trip. That is, if you can get out of bed the next morning.
Gardet subway station is 500 metres from the ferry terminal or there is a bus which connects the ferry terminal with the City Terminal, near the main railway station.
Stockholm Vartan terminal, Sodra Hamnvagen 46.
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