spontaneous/spontaneously

/spɒn'teɪniəs/　/spαːn'teɪniəs/　adj)いつもと違うことを急にやりだす

Be more spontaneous!

(マンネリのデートに飽き飽きしている彼女から彼氏へ）

I decided to go out tonight spontaneously.

unplanned  adj)無計画な

It was unplanned, but she decided to go out tonight.