It's ON! Ben Affleck spent another evening with 22-year-old Playboy model Shauna Sexton, on Sunday -- taking her to his favorite place, the Jack In the Box drive-thru, for a romantic date!

After the new couple's Thursday debut at Nobu Malibu -- being careful not to be photographed together -- they're not all in, letting X17 photographers come right up to their car window to snap away, while Ben and Shauna smiled and laughed together inside the car.

So here's how it all went down ... Ben's ex Lindsay Shookus was so pissed off at Ben's blatant cheating, she dropped off his Bentley in a Santa Monica parking garage on Saturday and drove away with a friend. Yep, she left his $350,000 ride in a generic garage with the keys in it. So Sunday, Ben sent his assistant to go fetch it and bring baby home and now the Mulsanne is back in the driveway of his Pacific Palisades manse.

And parked right next to it yesterday was Shauna's electric blue $20,000 Acura TSX that she rolled up in around 4pm, wearing sexy Spandex workout gear. Ben had the genius idea to go to the drive-thru for an early burger dinner and Sexton went along for the ride. The couple headed out in Ben's Range Rover to the Santa Monica hot spot fast food joint. The returned home together (presumably Ben doesn't eat fries in his red-and-black interior'd Rover), dined by candlelight and Ms. Sexton decided to spend the night.

Monday, 6:30am, Lindsay's blue Acura is still in the driveway ...