"She came in around midnight with a few of her girlfriends," a source says. "She didn't get crazy or anything."
While there, Lohan got a blast from her romantic past: "Wilmer [Valderrama] was there too," the source says. "They talked for a few minutes, but that was it."
Earlier in the day the actress, dressed in jeans and a Led Zeppelin t-shirt, made a brief appearance at the Rock & Republic 5th Anniversary Party in Santa Monica, where she smoked and chatted briefly with actor Lukas Haas.
The appearances mark a return to the Hollywood social scene for Lohan, who quietly turned 21 on July 2, and has been undergoing treatment at Promises, a rehabilitation center in Malibu.
On Saturday, one day after leaving the center, Lohan made her way to Sin City. While in town for a pal's birthday, she "went shopping at the Forum shops," a source says. "She bought two pairs of Jimmy Choo shoes and an outfit at Dolce & Gabbana."
Dinner followed in a private dining room at Social House, where "she was in a great mood and looked great," the source added.
Lohan also headed over to PURE Nightclub, where she socialized on the main VIP stage, another source says. Lohan imbibed a sugar-free energy drink and water, and danced for a while before making her way over to the DJ booth.
Lohan gave shout-outs to The Hills stars Lauren Conrad, Brody Jenner, Whitney Port and Frankie Delgado before inviting the group to join her party, says the source.
She also received another surprise guest: Mindfreak star Criss Angel, who hung out with Lohan at her table until they both left at close to 5 a.m., according the source.
Lohan issued a statement through her rep over the weekend saying she has "made the decision in support of her sobriety to wear an alcohol monitoring bracelet. In part she is wearing the bracelet so there are no questions about her sobriety if she chooses to go dancing or dining in a place where alcohol is served."
The rep, Leslie Sloane, also said: "Lindsay is working hard on her sobriety and we are all supporting her."
On Saturday, Lohan was seen wearing an alcohol-detection bracelet above her left ankle. The device works by sampling a person's natural perspiration every hour to determine whether the wearer has been drinking. The bracelet is worn 24 hours a day and a silent alarm signal is triggered if it's tampered with.