Robin Hood and his gang plan to kidnap the new tax collector
but their plan goes awry and instead they see the collector's son.
Tuck: How about a spot of kidnapping? Spontaneous kidnapping?
Brighten up the day.
Robin: Get him on the horse.
"spot of" means a period of time doing some activity. It doesn't specifically mean that the activity is spontaneous, though it does tend to sound as if it's done informally or for recreation: "We went down to the river for a spot of fishing". It's normally too light-hearted a phrase to be applied to something like kidnapping, but here it's obviously used for effect.

"spot of" can also mean a smallish quantity of something: "Do you fancy a spot of lunch?".
Thanks, Mr Wordy!