sideline vs spare-time activity

sideline and spare-time activity both are nouns.

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

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

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

sideline (noun)	spare-time activity (noun)
an auxiliary activity	an auxiliary activity
an auxiliary line of merchandise
a line that marks the side boundary of a playing field

sideline (verb)	spare-time activity (verb)
remove from the center of activity or attention; place into an inferior position