They took a specially chartered red London bus a short quarter mile from their palace home to the High Street Kensington station.
There, they met and chatted with charity workers who were selling the traditional red poppy emblems Britons wear to commemorate casualties of war.
William and Kate, both 31, were helping to promote a Royal British Legion bid to raise £1.6 million (or $2.5 million) for the charity in a single day in the capital.
The couple, who had left aides watching their 3-½-month-old son, Prince George, sleeping at home in Kensington Palace, had earlier greeted Royal British Legion staff, helpers and supporters outside their apartment at the palace.
Comedy actress and charity supporter Barbara Windsor asked the couple about baby George, to which Kate, who had pinned overlapping poppies on her red L.K. Bennett coat, replied, "He's half asleep."
"He's behaving himself this morning," William added.
On the bus ride to the station, the Duke of Cambridge told volunteers he and his wife were happy to have now settled into their 21-room apartment at Kensington Palace. "It's quite nice being in one place," he said.
Carl Court / EPA / Landov
While chatting with a group of RAF men, she said that William missed flying helicopters but also explained that she's tried piloting a plane herself, following in the footsteps of her late grandfather Peter Middleton, an RAF hero.
"My grandfather flew planes. I've had a few lessons on fixed wing," she said.
William – who, as a youngster, had been taken on the tube by his mother, Princess Diana – pondered the pleasures of being crammed in with other passengers.
"It's very sociable," he said, "Although I suppose everyone's got their earphones in."
Five minutes later, as the bus arrived at Kensington High Street, William and Kate emerged to meet armed forces personnel and volunteers spending two weeks selling remembrance poppies there.
They chatted with 37-year-old RAF Corporal Steve Johnson, a rock guitarist from Newcastle, who was busking around London's tube stations to raise cash for the cause. "Can you play something now?" Kate asked before thinking better of it. "You'd better not. It's too crowded.
During the chaotic, 10-minute visit to the station, military personnel gushed to PEOPLE about the royal couple.
"They were both really nice," said LAC Michael Scannell. "They both looked really happy, and Prince William was having a bit of a laugh, holding our buckets to see which us had more cash."
Added Staff Sergeant Juliet Wheeler: "Kate looked beautiful. She is very slim, but beautiful and stunning. It was a great morale boost for us all."
With reporting by Monique Jessen