臨時起意 的英文怎麼說?

[1] spontaneous[2] at will
  do something all one wants
  to one's heart's content

spontaneous = 形容隨性的個性，也形容依著個性、率性，例如今天心情好想旅遊，說走就走，不擔心或考慮太多事情

snacks should not be given to them spontaneously = 點心不應該隨性的給他們

at will = 我想怎樣就怎樣，也就是 "隨興" 的原意

隨著興致而作的英文相當多，例如 do something all one wants 跟 to one's heart's content。