hobbyhorse vs plaything

hobbyhorse and plaything both are nouns.

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

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

hobbyhorse: 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 hobbyhorse include toy.

hobbyhorse (noun)	plaything (noun)
a child's plaything consisting of an imitation horse mounted on rockers; the child straddles it and pretends to ride	an artifact designed to be played with
a topic to which one constantly reverts