Neighbours in Clifton were left stunned when envelopes containing £100 were posted through their doors on Christmas Eve.
The letters included a message saying simply it was a 'random act of kindness'.
Emma Crane, 41, was at home on Thursday afternoon (December 24) when her family had one delivered.
The childminder, of Peacock Crescent, said: "There was a knock on the door, my son went to answer the door.
"There was an envelope on the floor.
"He looked outside and did not see anybody, I got an envelope which said 'Random Act of Kindness' and some money inside."
The mum-of-two described the gesture as "very overwhelming" and said whoever had posted the money had a "heart of gold".
"Clifton has got a great community spirit anyway, people like this are worth their weight in gold," she added.
She and her family later went to Belton House, in Grantham, and used some of the money towards the trip.
Cherie Haynes, 38, who lives in nearby Rivergreen, also described feeling overwhelmed by the heartwarming gesture.
She said "we just heard the letterbox go" and added she thought "it's probably one of those free newspapers".
One of her sons found their envelope on a doormat.
Ms Haynes said: "I realised it was a Random Act of Kindness envelope, so I opened it with no major expectations.
"There was £100. It just set me off crying. It's really uplifted the whole morale of the house.
"I have no idea whatsoever [who delivered the letter].
"It would be nice to be able to thank them personally."
