piece of cake vs picnic

piece of cake and picnic both are nouns.

Word	Noun	Adjective	Verb	Adverb
piece of cake	Yes	No	No	No
picnic	Yes	No	Yes	No

As nouns, piece of cake and picnic are synonyms defined as:

Other synonyms of piece of cake include breeze, child's play, cinch, duck soup, pushover, snap, walkover.

piece of cake (noun)	picnic (noun)
any undertaking that is easy to do	any undertaking that is easy to do
	any informal meal eaten outside or on an excursion
	a day devoted to an outdoor social gathering

piece of cake (verb)	picnic (verb)
	eat alfresco, in the open air