hobby vs plaything

hobby and plaything both are nouns.

Word	Noun	Adjective	Verb	Adverb
hobby	Yes	No	No	No
plaything	Yes	No	No	No

As nouns, plaything is a hypernym of hobby; that is, plaything is a word with a broader meaning than hobby:

hobby: a child's plaything consisting of an imitation horse mounted on rockers; the child straddles it and pretends to ride
plaything: an artifact designed to be played with

Other hypernyms of hobby include toy.

hobby (noun)	plaything (noun)
an auxiliary activity	an artifact designed to be played with
small Old World falcon formerly trained and flown at small birds
a child's plaything consisting of an imitation horse mounted on rockers; the child straddles it and pretends to ride