Way 1 · best default
I decided on the spot
You made the choice right there, in that moment, with no planning before.
Examples
I saw the flat and decided on the spot.
They offered me the job, and I decided on the spot to take it.
In a conversation
A: You bought a puppy? Since when did you want a dog?
B: I didn't! I saw him at the shelter and decided on the spot.
Common mistake
Not: I made an instantaneous decision.
Say: I decided on the spot.
"Instantaneous" sounds scientific. In speech, "on the spot" is what people say.