I travelled from Stockholm to Rostock in north-east Germany by a ridiculous route, via Helsinki and Tallinn. This is not in fact because I am bad at geography (though I am) but because it worked out both cheaper and more interesting than simply taking a train from Stockholm to Berlin.
The Tallink ferry from Stockholm to Helsinki is like a giant floating shopping mall. It is cleverly crafted so there is no way to get cheap food or entertainment, but lots of incentive for the literally captive customers to spend money. The trip through the archipelago is very pretty, and to be honest the fare was so cheap that even after I'd doubled it buying overpriced (though at least decent quality) food I couldn't really complain. I decided to have a small meal (very nice smoked salmon with a red onion compote), and then splash out on an unlimited dessert buffet.
Obviously being seasick is a bad thing, but I do find it a little disturbing to be on a ship so huge that you can't feel the motion of the water at all. The third class accommodation was more than adequate, although I discovered that when there is no natural light at all I can't do my trick of waking up at a pre-decided time. I ended up waking in a panic at 9:30, half an hour before I had to get off the boat. But that did mean I had about 11 hours of solid sleep, so the total darkness was good for something.
I should have allowed more time in Helsinki, preferably not on a Monday when all the tourist sights and museums are shut. As it was I just wandered around the city on foot and by tram (they have a special tourist tram which does a circuit of all the interesting bits). The impression I formed was positive; Helsinki is not as outrageously pretty as Stockholm, but it is certainly attractive and there's some lovely architecture, including a lot of very nice art deco. And people were universally friendly and helpful (and anglophone!) Like a lot of tourists, I went there expecting to see moomins, but I was quickly overwhelmed by the amount of moomin-themed tat on offer. The last straw was discovering moomin-branded sanitary towels...
I took a short crossing to Tallinn the same day, which is why my time in Helsinki was so short (it didn't help that the ferry port is in an industrial area miles from any kind of action). Stayed in a middle-range hotel because that's affordable in Estonia and because it was sandwiched between two nights on boats so I wanted some decent sleep. I've been to plenty of pretty Mediaeval cities in my life, but Tallinn just takes the cake. The old city is so ridiculously, unreasonably photogenic it almost seems unreal.
In some ways Tallinn is ideal for tourists; everything you want to see is in a tiny area, and the people are charmingly foreign and very eager to take your money. There are cute little handicraft shops and restaurants selling traditional cuisine on every corner. I complained that the place felt like a theme park, but if it is a theme park, it's a really excellent one. It's still a little bit off the beaten track; it's cheaper than more obvious tourist destinations, and the various hawkers and vendors don't have more than a few words of English (their accent when they do try to converse is really very pretty!)
But the stuff calculated to appeal to tourists like me is being gradually edged out by the stuff calculated to appeal to the stag night crowd, who after all spend more money than the tourists out for authentic local culture. As soon as you leave the very centre of the old city, you are in a kind of post-communist hellhole of dilapidated buildings and ugly socialist architecture, interspersed with very new shiny casinos, strip joints, Irish pubs, booze supermarkets and chain hotels. I wouldn't have wanted to spend more than a day there, but one day was very well worth doing.
I visited a museum of Estonian history, where I discovered that Estonia has barely been independent for a century in total in the last millennium. That makes it particularly surprising that they have kept such a strong sense of culture, even preserving a Finno-Ugaric language (I would guess from looking at signs that Estonian and Finnish would be mutually comprehensible) through years of Prussian, Swedish, Lithuanian and Russian rule. I also ate sea buckthorn flavoured icecream, which was novel though not the tastiest thing I have ever eaten.
The ferry from Helsinki to Rostock might have been a mistake; the on board facilities were a lot less lavish than for the Stockholm to Helsinki leg, and I was pretty much bored, hungry and uncomfortable for 26 hours. (It didn't help at all that the only sensible place to sit had a really, really bad guitarist in it playing a very small repertoire of covers of Country and Western songs in a way that made it obvious that he had learned the lyrics by ear and didn't have enough English to actually parse where word breaks should be.) Well, I got to Germany for less than €150 total, including a day in Helsinki and a day and a night in Tallinn, so I can't really complain.