hobby vs spare-time activity

hobby and spare-time activity both are nouns.

Word	Noun	Adjective	Verb	Adverb
hobby	Yes	No	No	No
spare-time activity	Yes	No	No	No

As nouns, hobby and spare-time activity are synonyms defined as:

Other synonyms of hobby include avocation, by-line, pursuit, sideline.

hobby (noun)	spare-time activity (noun)
an auxiliary activity	an auxiliary activity
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