Sources tell PEOPLE the couple stayed in the tower's $40,000-a-night, two-story Hugh Hefner Sky Villa on Saturday, where a Jacuzzi pool, a glass elevator, an 8-ft. rotating bed and a full bar are among the amenities.
"They seem very happy together," says a source who spotted the two, who started dating in the past month. Hotel officials had no comment.
The trip was Spears's first appearance in Las Vegas since hosting a much-talked-about party at Pure nightclub on New Year's Eve – where, her rep told PEOPLE, she fell asleep.
It was a different scene Saturday night. Spears and model-actor Cohen, both 25, danced and smoked cigarettes at 8½, a gay club off the Strip, before leaving after about an hour.
Sunday night was even busier. After dinner at Social House and taking in a performance of Cirque du Soleil's Zumanity, the couple fueled up on Red Bull and then hit the Coyote Ugly bar. There Spears hopped on the bar for a dance, and the crowd "was going bananas," an onlooker tells us.
But after that, Cohen slipped back into the hotel while Spears moved on to Pure with several of her dancers and revisited her VIP table from New Year's Eve. She sipped Dom Perignon, danced to Madonna and her own songs, and stayed till about 3:30 a.m.
This is the second weekend in a row that the couple have been seen together. On Jan. 6 they hung out on a powerboat off Marina del Rey, outside L.A., and the next night met up at the W Hotel's Whiskey Blue bar in Westwood.