Justin Bieber and Selena Gomez's Slice of Sin City

What did Justin Bieber do with girlfriend Selena Gomez after he picked up six Billboard Music Awards last night in Las Vegas?

Yeah, he smooched her but then the two hit the Strip. No, the underage superstars didn't get their party on at some boozy bash. Instead...

Bieber and Gomez, along with their two bodyguards, stopped by Jaleo Restaurant at the Cosmopolitan hotel for a bite to eat. The jet-setting singers enjoyed slices of pizza still dressed up in their high-end awards show duds, according to onlookers.

Also at the Cosmopolitan, Rihanna joined Millennium Award winner Beyoncé and hubby Jay-Z at Greek eatery Milos, where the threesome dined just one table away from U2 frontman Bono.

In other celeb sightings news...

PARTYING: Joe Jonas hit up the Venetian's Tao Beach for some tanning time before hosting a pre-Billboard Awards show party at Tao nightclub, where he mingled with fans and hung out at a VIP table. After the awards, he partied at Lavo... Adam Lambert and boyfriend Sauli Koskinen enjoyed each others' company at Voyeur in West Hollywood for the J. Molinari jewelry launch while sipping Hpnotiq Harmonie cocktails together.

DINING: Green Lantern hunkster Ryan Reynolds pulled up to Fig & Olive Melrose Place in L.A. on his motorcycle...Newly single Jennifer Love Hewitt lunched with a group of gal-pals at L.A.'s Casa Vega Mexican restaurant the same day as Vivica A. Fox...Neil Patrick Harris had dinner with friends at Kiwame in Studio City.

Sexy Celebs including Paris and Nicky Hilton Rothschild, Kellan Lutz and Ryan Phillippe helped Orlando Bloom's lovely lady Miranda Kerr and her fellow Victoria's Secret angels celebrate the Sixth Annual What Is Sexy? List at L.A. hot spot the Beverly.

SWEATY: Alexander Skarsgård worked out at Equinox in West Hollywood at the same time The Big Bang Theory's Johnny Galecki was there doing some cardio on a treadmill.

BIG APPLE: Julianne Moore hitting a performance of the Tony-nominated production of AIDS-era play The Normal Heart.