WordNet sense 1 (a person lacking intelligence or common sense):

simpleton

WordNet sense 2 (exhibiting childlike simplicity and credulity):

childlike, wide-eyed, dewy-eyed, round-eyed

WordNet sense 3 (marked by or showing unaffected simplicity and lack of guile or worldly experience):

naif, naive

WordNet sense 4 (lacking complexity):

uncomplicated, unsophisticated

WordNet sense 5 (of a leaf shape; rounded at the apex):

obtuse

WordNet sense 6 ((botany) of leaf shapes; of leaves having no divisions or subdivisions):

unsubdivided

WordNet sense 7 (lacking mental capacity and devoid of subtlety):

half-witted, dim-witted, simple-minded

WordNet sense 8 (apart from anything else; without additions or modifications):

bare, mere

WordNet sense 9 (not elaborate or elaborated; simple):

plain

WordNet sense 10 (easy and not involved or complicated):

elementary, unproblematic

WordNet sense 11 (posing no difficulty; requiring little effort):

easy