When going to the Baltic you must take a full day tour in St Petersburg to see the winter place and many other intresting points during your tour.
We found the best currency to take was mainly Euros, however in Russia the US$ Dollar was the more acceptable currency.
In the maket place in Russia you can barter with the market traders.
Also it is also very easy to see the sights of Estonia on your own as there is or was a free shuttle bus form the docks into town.
The only other currency you will need is for Norway where the currency is Korner
All these currencies can be obrained on board the ship
We found the two most expensive places were Finland and Norway.
Depending where the ship docks it can be a ver short walk into town.
In Helsinki there was a free shuttle bus form the docks to the city centre, in the city centre we purchased tickets for a sight seeing tour of Helsinki board an old route master london bus there was a full commentary in English.
The tour starts and ends in the same place so you cannot get los.
We found when we were in Norway they accepted Euro's along with every other place we visited including St Petersburg, shop around at home rather than on the ship to get a more favourable rate.