accidental unintentional
odd, stray, unplanned
unplanned off-the-cuff impromptu
Haphazard is another word for random. Additional synonyms include arbitrary, indiscriminate and unplanned.
Unplanned.
This side trip was unplanned, but enjoyable.
ed
UNPLANNED
Random
Probably unknown.
This is likely the word "accident" (an unplanned or unexpected occurrence).
Impromptu is the closest to spontaneous word. It is a word representing sudden action.
Unanticipated, unexpected, random and happenstance to name a few.