Going between Sweden and Finland, we took the overnight cruise ships, which turned out to be pretty fun. I recommend going between Stockholm and Helsinki, though, rather than Stockholm and Turku. We did it both ways and the latter is a shorter trip with less time to really get the cruise ship experience, plus they roll you out of bed at 6 AM. We got a complimentary or accidental room upgrade or something on the Helsinki trip as well, which might color my perception of it a bit, but I still liked that trip better. We were on the Silja line, which means a giant blue boat, rather than the Viking line, which would be a giant red boat. Though perhaps there are other differences as well.
To imagine one of these cruise ships, take a hotel, a mall, a sauna, a casino, a nightclub, some restaurants and bars, and then moosh them all together and make the whole thing float. I don't know how many people they get on those things, but it's gotta be a lot. If you walk up on the deck, you get lovely views of all the little islands in the archipelago around Stockholm, or you can see midnight sunsets later in the evening. The rooms we had were pretty small, with beds that collapse into the walls, so you can decide between being able to lay down or turn around. I'm not sure how big and fancy the more expensive ones get, but our upgraded room did at least have a nice window overlooking the main "promenade" (i.e. shopping mall) of the ship. Wandering around the mall and restaurants and duty-free shops is interesting for a short while, but not for too long. Luckily, there was some other fun to be found.
There was a small stage and dance floor at one end of the first ship we were on. I poked my head in there a couple times during the evening, and noticed some show that seemed to have people in pirate costumes dancing to different sorts of music each time. Miriam and I went back later, though, and found a band playing and people dancing. They were mostly playing a bunch of "oldies," with some swing-able stuff early on, followed by more tangos, foxtrots, and slow dances later. We managed to get a decent little dance fix out of it. One really amusing moment came during a waltz when I was confused about why absolutely everybody suddenly wanted to dance in the same corner as us. Turns out these ships are so big it's easy to forget sometimes that you're actually out on the ocean. Until the dance floor tilts.
After dancing, we found a bunch of karaoke going on in another part of the ship. This turned out to be fascinating because the majority of the songs were in either Finnish or Russian (though occasionally English with one of those accents). It was really fun to hear pop music from other countries, and to watch all the foreign words going by on the screens. I tried to remember some of the lyrics long enough to write them down when we got back to our room. Now we'll see how well I did by googling for them and seeing if I can find the songs to listen to again.
I'll mention the ferry between Helsinki and Tallinn as well, as long as I'm here, though it was rather different, being more like a glorified bus and not a huge cruise ship. The one interesting thing about it was the music video showing on a screen when we boarded. It had a catchy little tune and this bizarre animation with hippo-dog things holding umbrellas. I really want to find that song again, and Miriam managed to jot down the main melody, but that's all we've got. After that one, they just showed American music videos from the '80's and '90's for the rest of the trip, so less interesting.