I have back-to-back July home exchanges scheduled; one in Stockholm and one in Copenhagen, two weeks in each city with one day in between. I plan on really immersing myself in the surroundings of each city but am considering a side trip and would like input as to whether to go to St. Petersburg to see the Hermitage for the art or take trains from Oslo to Andalsnes for the scenery. Does anyone have experience you can share regarding either of these? Thanks!
To go to St. Petersburg (even just for a day), you will need to get a Russian visa, unless you go with certain cruise excursions from Helsinki. The expense and hassle of getting a Russian visa is not to be taken lightly; do some research on this before you consider Russia further. If you are going to spend the effort and money to get a Russian visa, you will want to spend enough time there to make it worthwhile.
What Harold says is true. The visa will cost you a couple hundred dollars and you also need a letter of invitation to apply for the visa (a tour agency can do that for you). It takes a month or two for that paperwork. The visa process begins with an hour-ish long online questonaire. HOWEVER, St Petersburg is an amazing and beautiful city and the Hermitage is incredible. If you do get the chance to go (for more than a day) don't hesitate to do so. It's a fabulous city.
If you take the St Peter Line ferry, you don't need a visa for St Petersburg. You can stay 3 days / 2 nights if you travel from Helsinki. The whole trip from Helsinki takes four nights. The ferry (overnight) leaves every other day, so you need to check the timing. I did this last year taking the Viking line ferry between Stockholm and Helsinki. Alternatively, you can take a four night cruise on the Saint Peter Line from Stockholm stopping in Tallinn, St Petersburg, and Helsinki, but this only gives you a day in St Petersburg which really isn't much.
How much extra time do you have total? i've driven from Oslo to Andalsnes, but you also need a couple of days to really explore the area.
For both, I'd check the transit options and base your decision on that.
If you want some shorter options out of Stockholm, consider Tallinn, Helsinki, or Gotland.
This thread has my post on the visa free option on St Peter Line:
I've ridden the Oslo to Åndalsnes train route half way, Oslo to Otta via Lillihammer three times. While scenic, the route pretty much follows a river valley. At least to Otta.
The Norway in a Nutshell combination of trains, boat and bus links river valley, mountains as it traverses the Hardangervida and fjord crusing arms of the Sognefjord, terminating in Bergen. If you are looking for scenery, the Nutshell tour is as advertised.