I'm a slob. I leave things lying around, and am not neat. I was like that since I was a small child, but I'm imagining a device to encourage me not to do that.
The Small Item Gathering Device is a large-dog-sized, very mobile robot, with a container. It runs around the house, and puts all unattended things into a container. By unattended, I mean not on a shelf, drawer, or within my immediate grasp. When the container weighs more than a certain amount, it takes it to a centralized container.
I would then have to look in the container for anything that it took, and resolve that this time, I would put it away where it belongs. Probably not the first time, but it would get real old, real fast, until the motivation was there.
This would be most effective as a social conditioning tool if combined with social engineering as well. Have it patrol your child's room, and if a toy goes in the centralized container more than three times, it goes to charity. (Since many children worldwide live in poverty, and would love a new toy.) This will teach your child to put their toys in the proper place, where the Small Item Gathering Device does not look.
As a side benefit, fewer things are lost. If they're not in their proper place, they're probably in the central container.