before one can say 'Jack Robinson', " at short notice, extempore
on the spur of the moment, on the spur of the occasion [Bacon]
at once
on the spot, on the instant
at sight
offhand, out of hand
straight, straightway, straightforth
forthwith, incontinently, summarily, immediately, briefly, shortly, quickly, speedily, apace, before the ink is dry, almost immediately, presently at the first opportunity, in no long time, by and by, in a while, directly.
Phrases
no sooner said than done, immediately, if not sooner