The King has been shown having a chuckle at cards sent by well-wishers, including one of a dog with the caption “at least you don’t have to wear a cone!”.
The 75-year-old monarch has been sent around 7,000 messages of support from around the world since his cancer diagnosis.
Newly released images and footage show King Charles looking through some of them at his desk in Buckingham Palace’s Belgian Suite.
He seemed particularly tickled by a card featuring an illustration of a terrier-like dog in a head cone, recovering from medical treatment.
Pets often wear plastic collars after an operation, to stop them aggravating a wound or stitches.
Other cards spread out in front of the King include one that reads “Your Majesty Get Well Soon”, while a number appear to be hand-drawn by children.
Many have related their own experience of cancer, with messages such as: “Chin up, chest out, remain positive and don’t let it get you down.”
“Never give up. Be brave. Don’t push your limits. Get Well Soon,” says one note from a child.
Those who’ve included their address will be sent a message of thanks.
The King was pictured reading the cards on Wednesday – the same day he met the prime minister for their weekly audience.
He told Rishi Sunak that he’d been “reduced to tears” by the public’s support.
The monarch was speaking to Mr Sunak in their first face-to-face meeting since it was announced he had a “form of cancer”.
“I’ve had so many wonderful messages and cards, it has reduced me to tears most of the time,” King Charles told the prime minister.
He added: “I hear there has been a lot more potential attention on those main, wonderful cancer charities, many of which I have been a patron of for years.”
The King had been mostly staying at the Sandringham estate in Norfolk following his first round of treatment in London.
Buckingham Palace announced on 5 February that cancer had been discovered while undergoing a procedure for an enlarged prostate.
The Palace has not released details about the type of cancer or the treatment.