Coming to Russia in summer time you should definitely taste a traditional Russian beverage known as kvas. It is a fermented drink made from black rye or rye bread. It contains up to 1,2% of alcohol though is not considered as alcoholic beverage. That is why even children are permitted to consume it. It is also used as a major ingredient of summertime soup known as okroshka. The origin of kvas goes back five centuries to the start of beer production. In Eastern Europe it has been a common drink affordable by all strata of society . The word kvas is from the word "kvasit" which means to ferment, to make sour.
In warm summer kvas is a proper drink to slake your thirst in your Moscow city break.