Russian folklore and fairy tales feature a number of characters known by every kid, so I thought it might be an interesting idea to have a set dedicated to them. These characters are also known in other Eastern European countries, although names and spelling may vary.
I suggest a set of 6 stamps:
Баба Яга (Baba Yaga)
The main female antagonist. She flies around in a mortar, wields a pestle, and dwells deep in the forest in a hut usually described as standing on chicken legs.
Иван-царевич (Ivan Tsarevich)
Sort of "Prince Charming", saves damsels in distress and defeats evil.
Кощей (Koschei)
The main male antagonist. An evil sorcerer, who is kept immortal or deathless by a spell.
Змей Горыныч (Zmei Gorynich)
Three-headed dragon with a thing for kidnapping women.
Василиса Прекрасная (Vasilisa the Beautiful)
Not exactly a stock character, but probably the best-know female protagonist.
Жар-птица (Firebird)
Pretty much a phoenix, whose feathers posses magical qualities.