Quail love textures. By textures, I mean things that they can feel - under their feet and beak. They use their beak to explore new things, and if they like it, they get their feet in there.
In the wild, quail have two normal behaviours that relate to this - scratching the ground with their beak and feet to search for morsels of food, and scratching in sand/dirt substrate in order to dust-bathe, which feels good and helps keep them free of external parasites. Both these behaviours are innate, meaning they are instinctive at birth. I have seen quail chicks only hours old "dust-bathe" under the comforting heat of an infrared lamp.
In this video I give Brita and Blondie two objects: a folded terrycloth towel and a flattened plastic grocery bag. They discover the towel immediately and begin pecking and scratching at it. Eventually they begin dust-bathing in it - which is rather counterproductive, since there isn't anything to "bathe" in - but when quail are really happy and feel really good, they dust-bathe, regardless of whether they have substrate. When I used to keep large numbers of quail I had them in large 10-foot cages with wire mesh floors to allow the droppings to fall through, and during their evening meal, some birds were so happy to be eating that they would dust-bathe between mouthfuls on the bare wire.
When I take the cloth away, both birds look a little perturbed for a moment but eventually go to explore the bag. Blondie gets right in there with the same enthusiasm, but Brita gets bored. When I replace the towel, Blondie abandons the bag and goes back to it. I have seen them seek out similar objects in my apartment - fuzzy slippers, laundry, etc. They seem to really like soft and squishable textures. They will also try to dust-bathe in any kind of substrate - fresh pine shavings, house-plant dirt, kitty litter boxes, etc.
I am starting to get a better idea of Brita and Blondie's personalities. Blondie is a lot more needy for Brita's company that Brita is for Blondie. If they are separated, Blondie will call again and again and ignore everything else around her, where as Brita will just carry on. Blondie, however, is usually the first to explore new objects and the most enthusiastic about them, and least likely to get bored. This makes me think that she might be the better candidate for clicker training.