Rihanna is having a great time on vacation, as evidenced by her sexy tweeted pics, but it's not all play. The singer is still sticking to her workout regimen while in Hawaii, even if it means doing it in her bikini.

The 23-year-old has been soaking up the sun since Saturday, and documenting her vacation via Twitter. But in addition to swimming with sharks and paddle boarding, RiRi is getting in plenty of extra cardio, too. Only instead of traditional workout gear, the 'We Found Love' singer is hitting the gym in a white-hot two-piece.

On Wednesday, Rihanna took to Twitter to share a photo of herself on the elliptical, wearing nothing but a string bikini and a pair of shades. Nope, not even a pair of sneakers for this songbird.

The Barbadian beauty has been serious about working on her fitness, with the help of trainer Ary Nunez, who recently told Shape magazine that the singer's regimen includes a variety of exercises.

To get Rihanna's heart pumping, the trainer puts her through a mix of calisthenics, martial arts and dancing, which we know the R&B star has down pat. “We focus on the core muscles, and we do a ton of repetitions,” Nunez says. Well, it sure looks like it's paying off.