ingenuous 

Definition:  (adjective) Lacking in cunning, guile, or worldliness.  
Synonyms:  artless  
Usage:  His ingenuous delight in it was a delight to her, and a new and mutual love-thrill was theirs—because of a flower.  

artless: simple, natural and honest

cunning: able to get what you want in a clever way, especially by tricking or cheating sb

guile: the use of clever but dishonest behaviour in order to trick people

worldiness:  the fact of having a lot of experience of life and therefore being difficult to shock