Rather than the big Las Vegas blowout she had once wanted, Lindsay Lohan quietly marked her 21st birthday Monday in rehab.

Lohan’s mother and siblings were photographed arriving at Los Angeles International Airport over the weekend for what her mother had said would be a family celebration.

And on Monday, Lohan was seen jogging on the beach during a break from her stay at Promises rehab center in Malibu, her latest furlough from the facility, which she entered last month.

Saturday, with a brand new iPhone in her hand, she stopped by the boutique Planet Blue, where rock music blares and shelves are filled with items for the bedroom, bathroom and clothes closets.

She was seen the day before, on Friday, with friends – a male and a female – looking great and listening to the piano at the Chateau Marmont. The star drank tea, diet coke, and water, according to an observer at the hotspot, who added: “She’s turned things around, at least for now.”

And last Wednesday night she attended an AA meeting in Santa Monica before meeting pals for dinner at Katsuya in Brentwood – with a stop at Promises in between.

“She came in with a whole bunch of friends, there were like seven of them, and she walked in incognito, super low-key,” a fellow diner says. “They sat down for some sushi and were just really enjoying themselves. She wasn’t really noticed at all and was just happy, in a good mood.”

The relatively quiet week was in contrast to her original plans for a party at PURE Nightclub in Las Vegas that was to be open to the public. “I’m going to milk it because it’s a big birthday,” Lohan said on The Ellen DeGeneres Show in May. But the bash was canceled earlier this month.