A quiet day at the beach for a big birthday.
The couple marked the occasion with a quiet bike ride down Gordon's Pond Trail, which was not closed to the public, according to the Secret Service, so onlookers who passed the Bidens by wished her well.
When the Bidens returned, a rendition of "Happy Birthday" broke out, drawing a laugh and a "Thank you!" from the first lady.
Jill Biden is the oldest sitting first lady in modern history but remains an active senior citizen.
She is also the first first lady to hold a paid job outside of the White House, continuing her longtime career as a community college English teacher. She's currently off for summer break.
Earlier, the president, 78, at multiple points, took a hand off the handlebars to salute, wave and point to those gathered.
One reporter shouted, "Did you enjoy your bike ride Mister President?" to which he simply responded, "Yes," with a slight smile.
The pair was surrounded by Secret Service agents, on and off bikes.
The president ignored shouted questions about what he got his wife for her birthday. She at least got decent -- if not perfect -- birthday weather, with temperatures in the 70s and partly cloudy skies in the beach town, still suitable for a bike ride in short sleeve shirts.
Michael LaRosa, a spokesperson for the first lady, said no family or friends were joining the Bidens on Thursday.
It's their first trip back to Rehoboth Beach since moving into the White House -- and the town seemed ready to welcome them back. A sign outside a local store wished her a "Happy Birthday" and a boardwalk ice cream shop named a flavor in honor of "Biden's Summer White House Cherry."
It's no surprise that the first lady would choose Rehoboth to spend her birthday. She's been candid that she pushed Biden to buy the $2.7 million home in 2017, after leaving the vice presidency and their signing an $8 million book deal.
She told Vogue magazine last year that Biden promised her, "when I write my book, I’ll buy you a beach house."
"I wanted it to be the kind of place where you can come in in your wet bathing suit and bare feet and I can just take the broom and brush out the sand," she said in the March 2020 article. "And that’s what this is. Everything’s easy."
Above the front door, the article said, is a plaque reading, "PROMISE KEPT."
Their vacation home is just blocks from the Atlantic Ocean and comes with a swimming pool overlooking Cape Henlopen State Park, where Biden's helicopter landed on Wednesday evening so he could join his wife for her birthday getaway.
But since all good things must come to an end, the Bidens are scheduled to return to Washington on Friday.