Well I was there in springtime, but in January/February I'm sure you can assume the weather would be about what it is like in the Northern US. Around 30F with snow and maybe some shitty rain days too. Keep in mind that Stockholm is really high in latitude, so while you won't get total darkness during the day or anything, it will still get dark a lot earlier than you're probably used to, as well as the sun coming up later.
Food in Sweden is quite a bit more expensive than you're probably used to (I'm not sure where you are from). If you guys buy your groceries in a store and cook it yourselves at hostels, you will save a lot of money; if not, you can count on spending 10USD on a hamburger and going broke in three days
As for metal-related stuff... There are a couple of really awesome record shops I went to. One is called "Sound Pollution", on
Gamla Stan (the island with all the really old buildings on it) on the street
Stora Nygatan. I forget the address though. It was pretty cool, lots of vinyl we're not used to seeing in the states, and of course a nice selection of CDs, many of them with rare/alternate artwork that is kind of hard to find over here. The other is called "Repulsive Records" and it is kind of northeast of the city center, I believe in the
Norrmalm district; I want to say the street was something like "St Eiriksgatan" but I can't really remember. Anyway, it's worth the T-bana ride up there to check it out. Nice selection, and the guy that runs it is really nice; if you want you can ask him to "preview" any CD (since the actual disc is kept beneath the counter), and he gets pretty pumped when you tell him exactly how far away you came from to go to his shop. Price-wise, both places are probably more than you're used to spending on CDs, but honestly not much more than you'd pay at say your average mall in the states. There are also your sort of regular run of the mill record/CD shops spread through the city as well, which are sometimes worth popping into if you see them, but not necessarily "planning" to visit, if you know what I mean. I stopped in one along the main pedestrian shopping street on a whim and got some nice Scandinavia classical music collection for like $2, as well as the single Blind Guardian had out at the time, and a few other more "popular" kind of CDs (I think like, Faith No More, Led Zeppelin) for a couple of dollars each.
There are a couple of places you can see shows while you're there too. Nothing much was going on when I was there (Dissection played their last show like 2 days after I left
) but you can ask around, maybe check with the guys at the aforementioned metal shops. I know there is a small little venue in the Slussen area (between
Gamla Stan and
Södermalm) but I can't for the life of me find out anything about it on the internets. It was there though, I definitely saw it
For souvenirs, well honestly every city in Europe is totally full of souvenir shops, so it kind of depends on
what you're looking to get, if I'm honest. Stockholm probably has hundreds of those sort of "run of the mill" shops run by Pakistanis selling a bunch of stuff with the Swedish flag on it made in China. Sometimes though you can find some legit things; for instance most places you'll see the quintessential dalahäst figurines, which are little painted horses. If I remember correctly, most of these are made locally (look for the sticker underneath...though I suppose they could just be *buying* the stickers to put on cheap mass produced ones...well whatever
). But also keep in mind there are tons of off the wall antique shops and such in Stockholm, so there's something for everyone, really.
As for other things to do...well the #1 sight is just the city itself. Stockholm is a beautiful place. Everything had just begun blooming when I was there so it was extra great, but I'm sure it would look nice in the winter too. The architecture is really nice and gives you that "old world feeling" if you know what I mean. But other than that, there are some world-class museums and stuff that you really should visit when you're there. The best one is called "
Skansen", which is kind of "Sweden in miniature"; they took traditional buildings from all over the country (some like 1000 years old) and brought them to this place in Stockholm. They also have native Swedish animals like moose, wolves, bears, elk, etc. You could even buy your souvenirs here as there is glassblowing and handicrafts all over the place. Only thing is I'm not sure what their hours would be like in the winter, but it's worth checking out. Another cool place is the "
Vasamuseet" which is a museum entirely based around this renaissance era war galleon that sank in the harbour on its maiden voyage a few hundred years back. It is wonderfully preserved and you can go in and walk all around it and stuff; pretty awesome if you're into that kind of thing. Other than that, there are probably hundreds of other museums in Stockholm for every possible interest area. There is a "Ship History Museum" with hundreds of little models of Swedish ships through the ages, an Army museum, a Music museum, etc etc etc... lots of these places are either free or almost free if you buy a "Stockholm Card" (this also covers your transportation as well, if I remember correctly).
For accommodation, you probably know this already, but hostels are the way to go. The two that I spent the most time at were called "City Backpackers" and "City Lodge", both kind of in the same direction north of the train station. One was really cool, and the other was just kind of average (neither bad), but I'm having trouble remembering which was which (it's been about 4 years). I *think* it was the former that was awesome, but I could be wrong! Either way, there are a ton of hostels in Stockholm, and you'll have no trouble finding a place to sleep, especially in the winter.
Anyway I guess that's enough to get you going. You can also check
http://wikitravel.org/en/Stockholm to see if there's anything I missed. I'd recommend visiting the city's website, and maybe looking at GoogleEarth a little just to get an idea of where some things are and whatnot. Enjoy your trip