Britney Spears made a surprise appearance at a fundraiser for Jenny McCarthy's autism charity Generation Rescue Saturday in Los Angeles.
As guests like McCarthy's beau and Charlie Sheen and wife Brooke Mueller made the rounds, Spears, 26, sipped on red wine.
Carrey's daughter Jane also belted out a few Frank Sinatra and Jackson 5 tunes.
Spears, 26, "definitely looked happy," a witness tells Usmagazine.com. "She was clapping and smiling a lot."
See photos of Britney Spears in a bikini.
It was Spears' first public outing since reaching a custody agreement with ex Kevin Federline last week.
While Federline gets sole custody, Spears gets three visits a week from Sean Preston, 2 1/2, and Jayden James, 1 1/2, including two overnights.
See photos of Britney Spears and her boys.
If all goes well, she could have her boys roughly 40 percent of the time, her attorney, Laura Wasser, has said.
Wasser told the Associated Press the "difference" in Spears "from the beginning of this year to now is astonishing."
Spears is spending her summer recording a new album.
