spur-of-the-moment
Occurring very suddenly, impulsively, and/or without preparation beforehand; impromptu or capricious. You've got to stop making these spur-of-the-moment decisions about your business, or you'll end up running it into the ground! It's totally out of my character to take a spur-of-the-moment trip like this, but I just needed to get out of the city for a while.