I never buy any currency in advance of my trip. I typically just hit one of the ATM's in my arriving airport and after departing from the ship in a port if I need more. The total cost of the transaction is competitive with any retail exchange rate and a whole lot easier.
For the Baltic trip Euros and credit cards work for many things including buying tickets on the various Tourist Buses that run in most if not all the citiies you are visiting.
I think it is a waste to keep converting money for a 6 hour visit when you can mostly get by with Euros - preferred by over the Ruble in Russia. Same in Estonia and Poland.
Bottom line, you only need Euros, Pounds, and a credit card for this this trip. Get rid of your remaining money in duty free on the way home and you'll never have to visit a single money exchange window on the whole trip!