WordNet sense 1 (not wise in the ways of the world):

unworldly

WordNet sense 2 (marked by or showing unaffected simplicity and lack of guile or worldly experience):

naif, naive

WordNet sense 3 (lacking complexity):

uncomplicated

WordNet sense 4 (having few parts; not complex or complicated or involved):

simple

WordNet sense 5 (awkwardly simple and provincial):

hick, rustic, bumpkinly