on the spur of the moment

"on the spur of the moment" = without planning or preparation

"I decided to go to the party on the spur of the moment, so I threw on some clothes and left."
- She always makes decisions on the spur of the moment, so you never know what she's planning.

On the spur of the moment we drove up to Cambria on Sunday to see the elephant seals. It was prime season. The moms were just finishing up nursing the pups, the bulls were challenging each other, and some mating was going on too.  source