A friend of mine did the crossing to Stockholm, said it was a party all night long. She was seasick, so didn't enjoy it much, though! We looked into taking the boat as well, and I gather you have the choice of cabins or just open seating. In the summer, people just hang out on the decks, but obviously in winter you're not going to want to do this. All reports I've heard suggest that if you wish to drink and party, you'll have company.
The trip from Helsinki to Tallinn is only about 1 1/2 hours by the hovercraft, so it's not much of an event. More like a bus ride than a cruise. The Finns come over to Tallinn and the rest of the Baltics to buy cheap booze, just so you know.
Tallinn is a great city, even in Feb, which is when we went. If you're going to be there for a few days, consider getting the "Tallinn Card". For one price you get admission to all the museums and sights, and though we don't usually take bus tours, we enjoyed the complimentary city bus/walking tour. It was especially nice in Feb, because we got to see a lot of stuff out of town that we wouldn't have on our own, because it was too cold to walk around and wait for buses, etc. Also, the Tallinn Card covers public transit, which within the city is quick and efficient. We got the three day pass and it paid off after about a day and a half of moderate sightseeing/transit and the bus tour. Plus you get some discounts at restaurants. When in Tallinn, look for the "Tallinn in Your Pocket" (usually you can get them at the newstand kiosks all over town, the bus depot or at a big bookstore - probably also at the port.) The restaurant and accommodations listings are comprehensive, humourous and sometimes scathing. Believe the restaurant reviews, especially the bad ones. They also list a lot of clubs and cultural events, and pubs that have live music etc. Check out
www.inyourpocket.com for a preview, but buy the book in country - it'll be up to date and is much more comprehensive than the online info.
In Stockholm, a great hostel is the Af Chapman. It's a two part hostel, there's a hotel type building on land, which is reception and where the breakfast is served, and there's a 18th C. sailboat converted to dorms a few steps away on the harbour. We stayed on the boat - in January. The boat was warm and there was hot water for showers, and there were a fair number of people staying there, every time we came through the common room there was a group hanging out. Best of all is the location - it's right across the bay from the Gamla Stan, about a 5 - 10 minute walk, depending on your pace. There's also a bus that drops you off about 150 m from the hostel, which is handy with bags! We were awakened about 5 one morning by a strange booming sound, which, it turned out was ice banging against the hull as it swept past.
We also got the Stockholm Cards there, but we were pretty aggressive about hitting museums and such - I think we went to 15 in three days. They were all pretty cool though, the Wasa museum (it's a boat, not a cracker) is awesome, as is the medieval museum. We all enjoyed the Natural History museum as well, though that's kind of out of the way. The Maritime Museum and Technology museum are near each other, but again a little ways out of town. Again, free transit with the card, but I'm sure you could just buy a pass if you don't want to get the card.
