To kick off Friday night's celebration, Spears, 30, and her former agent fiance had dinner the Sugar Factory American Brasserie, where they feasted on salmon and the eatery's famed chocolate pizza.
Just before midnight, the duo joined 30 family members and friends for dancing at Chateau nightclub, where Spears presented her man with a two-tier red velvet birthday cake.
"She seemed really excited to be at the club," an insider tells Us Weekly of the soon-to-be Mrs. Trawick, who debuted her three-carat Neil Lane ring for the first time Friday night at the bash, which doubled as an engagement celebration for the pair.
"Tonight was one of the most magical nights of my life," Spears tweeted after Friday's festivities, that took place one day after Trawick surprised her with a marriage proposal.
"We have such a great time together!" Spears has said of her guy. "He makes me laugh, and we are so comfortable being ourselves. I've never been happier."