Synonyms of informal

1 Syllable
free, frank, cool
2 Syllables
mellow, loose, motley, folksy, chatty, open, outdoor, outgoing, hilltop, sportiest, homey, breezy, mixed, sportier, common, sporty, plebian, improv, chattier, easy, yarder, offhand
3 Syllables
spontaneous, straightforward, hang-loose, outgiving, easygoing, relaxed, natural, populist, familiar, nonformal, congenial, colloquial, unfussy, urbane, homelier, commonest, homely, throwaway, off-the-cuff
4 Syllables
vernacular, unselfconscious, democratic, intimate, vernaculars, everyday, ordinary, unrepressed, unofficial, extempore, low-pressure, free-and-easy, inconspicuous, unrestrained, unconstrained, unself-conscious, unhampered
5 Syllables
spur-of-the-moment, extemporaneous, conversational, uninhibited, unceremonious, unconventional, nature-loving, extemporary
6 Syllables
confabulatory, improvisatory