— It’s not a problem one faces every day: What kind of present do you give a nonagenarian who lives in castles and is arguably the world’s most famous royal?

The thick crowds that lined the streets in Windsor on Thursday, where Queen Elizabeth II was celebrating her 90th birthday, opted for flowers (lots of carnations, said to be the monarch’s favorite), cakes (one shaped like a corgi), cards (many homemade) and a giant stuffed bulldog.

Windsor, a city about 20 miles west of London, was the focal point for the queen’s many birthday celebrations taking place across the country.

Wearing a bright, lime-green coat dress and hat — she likes to be spotted in a crowd — the queen mingled with thousands of well-wishers during a 30-minute walkabout. She cheerfully accepted dozens of cards and flowers — which were passed to her lady-in-waiting — and unveiled a plaque marking “The Queen’s Walkway,” a 6.3-kilometer walking trail.

“At 90, she is still doing this. I think she’s just wonderful,” said Jeanette Standee, 64, a self-described royalist who made the queen a birthday card and wears a watch whose face sports a picture of the monarch.

There is a much more festive mood in the air than there was in September, when Elizabeth II became Britain’s longest-reigning monarch, surpassing her great-great-grandmother Queen Victoria. At the time, royal-watchers said she was reluctant to mark that milestone, in part because her reign began with the death of her beloved father.

But there was an undeniable lightness on this sunny spring day while throngs sang “Happy Birthday” and snapped pictures on their smartphones as the queen walked by.

At one point, she stopped to talk to “The Great British Bake Off” winner Nadiya Hussain, who showed her the orange drizzle cake she whipped up for the occasion.

Elizabeth II has spent nearly two-thirds of her life as head of state, and she’s still carrying out royal duties. This week, she marked the 500th anniversary of the postal service and opened a bandstand. On Friday, the queen and her husband, Prince Philip, will host a lunch for President and Michelle Obama.

“Most people at 90 would want to kick back, I think,” said Mary Laturner, 32, an Australian living in London who was sharing a royal-themed birthday cake — retailers are keen to cash in on the occasion — with others in a dense crowd outside Windsor Castle. “She’s an ambassador for what it means to work hard regardless of your age.”

The queen is turning 90, and throngs of people line the streets outside Windsor Castle in the hopes of catching a glimpse of the monarch on her walkabout. (Karla Adam/The Washington Post)

Back in London, British Prime Minister David Cameron led tributes in the House of Commons, praising the monarch as a symbol of continuity.

More tributes rolled in. Actor Roger Moore, best known as one of the James Bonds, told the BBC that the queen was “calm” and “beautiful.” He admitted he was a tad jealous when it was Daniel Craig, another James Bond, who got to make an Olympics skit with her.

The award for tiniest tribute went to scientists at the University of Nottingham who used a focused ion beam to etch the message “Happy 90th Birthday Your Majesty” onto a strand of corgi hair.

Prince Charles, the queen’s eldest son and heir, recorded a special radio broadcast of an edited passage from William Shakespeare’s “Henry VIII.” In the evening, he hosted a private family dinner.

At a second public appearance, the queen lit the first of over 1,000 torches or bonfires set ablaze across the U.K. and beyond.

Gun salutes thundered out across Stirling, Edinburgh and London.

Her birthday was also marked with the release of photographs taken by photographer Annie Leibovitz. In one picture -- splashed on the cover of the Daily Telegraph newspaper -- the queen is photographed with her two youngest grandchildren and five great-grandchildren, including Prince George, who calls the head of state “Gan Gan.”

But for some, it was important to be here in Windsor, a city dominated by the massive Windsor Castle, a royal residence and the queen’s weekend home.

Judy Daley, 50, left her home town of Cardiff at 3 a.m. so she could plant herself in the front row, where she hoped the queen would take notice of her giant pink balloons.

“Every day she is out performing royal duties. At the age of 90. She’s a formidable lady,” she said.

Locals turned out, too. “My queen, that is,” said 80-year-old Windsor resident Anne Pamment as she pointed at the queen and Prince Philip as they drove by in an open-topped Range Rover, dubbed the “Queenmobile” in the British press.

At that moment, the queen turned in her direction and gave a tiny, white-gloved wave.