Fitness In Helsinki

Hi!
I am a member of InterNations in Sofia, Bulgaria. I am going to be visiting Helsinki for about a month in December and wondered if anyone could recommend a gym/fitness centre in the city centre (somewhere in the area around the main station preferably) where you can pay per session instead of having to pay out for membership.

I am also interested if anyone knows of any yoga classes that operate on the same basis (English-speaking would be perfect but if it is in Finnish then this wouldn't be a problem either).

Thanks and best wishes
Jo