WordNet sense 1 (occuring from time to time):

occasional

WordNet sense 2 (contrary to rule or accepted order or general practice):

irregular

WordNet sense 3 (occurring or appearing or singled out by chance):

accidental, chance

WordNet sense 4 (without apparent forethought or prompting or planning):

unplanned

WordNet sense 5 (characterized by a feeling of irresponsibility):

fooling

WordNet sense 6 (natural and unstudied):

free-and-easy

WordNet sense 7 (suited for everyday use):

everyday

WordNet sense 8 (not formal):

informal

WordNet sense 9 (not showing effort or strain):

effortless

WordNet sense 10 (marked by blithe unconcern):

insouciant, nonchalant

WordNet sense 11 (lacking in interest or care or feeling):

unconcerned

WordNet sense 12 (hasty and without attention to detail; not thorough):

cursory, passing, perfunctory