This side trip was unplanned, but enjoyable.
This is unplanned.
Unplanned.
You can use "unplanned" in a sentence like this: "The unplanned detour took us through a beautiful countryside that we had never seen before." This showcases how something unexpected can lead to a positive experience.
ed
UNPLANNED
accidental unintentional
Random
odd, stray, unplanned
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.