This side trip was unplanned, but enjoyable.
This is unplanned.
Unplanned.
ed
UNPLANNED
accidental unintentional
odd, stray, unplanned
Random
Probably unknown.
unplanned off-the-cuff impromptu
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.