And clues to the locations will be posted on social media.
The envelopes will have a logo which reads “We Make Good Happen” and inside will be instructions explaining to the lucky finder what they should do.
The only rule is that they MUST spend the £10 on somebody else.
Video director Matt Callanan, from Roath, Cardiff, is on a mission of goodwill.
The generous gift giver has set himself a challenge to carry out 403 good deeds – and this is one of them, Wales Online reports.
“I will put some envelopes out around well-known places and in some less obvious places because I want people to get a bit of an adventure,” he said.
“I will put instructions in the envelope and ideas of what they could do with the money.
“I will ask people to report back using a hashtag. The only rule is that they can’t spend it on themselves.”
Matt suggests that lucky cash hunters could buy flowers for a partner or friend, buy lunch for a homeless person, buy baking ingredients and make a cake to distribute to a neighbour or buy a gift for an elderly person.
The idea of setting himself the challenge came to Matt after he met his all-time hero, Bill Murray, at George Clooney’s house.
Filming a documentary at the time, after leaving the house starstruck and feeling lucky, Matt decided to stop on the way home to buy a scratchcard.
He won £20, which he spent on a man selling the Big Issue in Cardiff Bay.
“I bought the scratchcard because I thought to myself ‘could this day get any luckier?’ and I thought that if I was ever going to win, that would be the day,” he said.
“I had the lucky £20 and I took the man shopping for food and some essential items. He gave me a massive hug.
“He was really happy and I was happy and I thought there was something great about doing a good deed so I decided I would try to do 100 good things.”
When his wife Andrea told him 100 would be too easy, Matt looked for a higher number.
The 403 is a figure from his favourite film, Groundhog Day, where Bill Murray spends 12,403 days stuck in a repetitive hell.
So far Matt has done 50 good deeds, including designing a poster for a girl who was body shamed on the way to her prom, giving a family a lift after they became lost, buying new wheel hubs for his wife’s car, paying for a family to go swimming and clearing an elderly neighbour’s garden.
Matt said: “I would like to stretch it further to the full 12,403, but I want to show that it’s possible to do these things if you want to.”