The couple were joined by Spears's two sons Sean Preston Federline and Jayden James Federline on a trip to Sin City, with People reporting that they stayed in a "mega suite" at the Planet Hollywood hotel and casino.
Spears and her fiancé toasted Trawick's 40th birthday with Planet Hollywood chairman Robert Earl, but a source said that the popstar "wanted it to be quick because her kids were upstairs and she wanted to put them to bed".
The happy couple then had dinner with family and friends at Sugar Factory's Chocolate Lounge and partied at the Paris hotel's Chateau Nightclub until 1am local time.
An eye-witness revealed: "Britney seemed happy, holding Jason's hand."
Spears later took to Twitter to share her joy with fans by writing: "Tonight was one of the most magical nights of my life. Such an amazing time with my new fiancé and our closest friends and family."
She first hinted at the engagement on Friday morning, when she tweeted that Trawick had given her "the one gift [she's] been waiting for".
The 'Criminal' singer previously tied the knot with childhood friend Jason Allen Alexander, but their union was annulled just 55 hours later.
Spears was married to dancer Kevin Federline for two years, but their relationship ended in divorce in November 2006. Federline is the father to Spears's two sons.