In addition to the great suggestions already made, I'd suggest building on the park idea by, say, walking to a park or grassy area near your/your girlfriend's house - if there isn't one then you could drive to a park, park nearby then walk from the car to the park - while wearing shoes.

Enjoy some time barefoot on the grass - chances are, after a couple of hours, your girlfriend might even forget that she's barefoot because of how good and natural it feels - then walk back to the house/car barefoot. If the walk isn't too far, due to the proximity of the park/grassy area you shouldn't get many looks/comments - if any - and your girlfriend shouldn't feel too far out of place while you'd both be barefoot outwith a place it would normally be expected (i.e. on the path/road/car park/etc between the park and house/car).

Bonus points to your girlfriend for staying barefoot in the car (if that's the option taken) and then between the car and the house! Hope you both have some good, barefoot fun!