Spears treats Federline to a Vegas blowout for his 28th

Britney Spears threw husband Kevin Federline a birthday party on Tuesday in his favorite city: Las Vegas, PEOPLE reports.

The singer and Federline, who turned 28 on March 21, arrived just after 8:30 p.m. at the restaurant Tao Las Vegas at the Venetian, where they slipped in through the back door and joined their 10 guests in a private dining room.

Spears (in a black halter dress with a low-cut back) drank a Cosmo, while K-Fed (in a white tank and a New York Yankees cap) went for a Jack and Coke, shots and beer. The group dined on sushi, chicken satay, lobster tempura and grilled Kobe beef, followed by cake served by two little people in tight dresses.

“Kevin was wearing his ring even though Brit wasn’t,” says a witness. “They were being affectionate with each other. She sat on his lap for awhile. He fed her a bite of an appetizer. ”

By 10:30, Federline had finished opening his presents and the party headed to Pure Nightclub at Caesars Palace, where Spears snuggled with the birthday boy on the club’s VIP bed when she wasn’t dancing barefoot to tunes spun by DJ Hollywood (she requested Prince).

“They didn’t leave each other’s side all night,” a clubgoer tells PEOPLE. “They danced together – Britney was doing all her famous dance moves. Toward the end of the night, they were all over each other – kissing and hugging, having a great time.” The couple finally called it a night at around 2 a.m.