Hey again Dave,
If you're that close to St. Petersburg definitely consider going. It's only a train ride away and in Summer it's a really cool journey.
St. Pete is one of the most beautiful cities in the world and if you're a fan of history and the arts it just doesn't get much better. You will need a visa (don't know how easy it is to get one in Helsinki, maybe a Finnish member can check this for you!!!! Kiitos) and it may be a bit of hassle, but worth thinking about.
There's plenty to see in Helsinki but I'm no expert. The Tourist Office down by the seafront (corner of Unionsgatan and Pohjoisesplanadi I think) are fantastic and can recommend so much, they may even be the best Tourist staff I've met, and how many languages they speak!
A quick list of places I might recommend are:
The Ateneum - National Art Gallery
Sit at a cafe on the Esplanadi and watch the world go by
A morning visit to the Kauppatori (Market by the harbour)
Take a boat trip around the islands
Get drunk in KarlieKaarle XII at Kasarmikatu 40 (www.kaarle.com) I REALLY MEAN THIS!!!
There's lots to do, just ask a Finnish person and they'll be glad to help.
One last thing. I found that when you walk the streets there doesn't seem to be many shops and bars but if you go through any door you'll find everything is housed behind the facades as it's too cold to walk the streets in winter.
www.finnguide.fi is a good place to get an overview.
have a great time man, and say hello to lovely Helsinki for me!