One of my dogs gets separation anxiety. She is fine if our other dog is with her but gets very upset if he is gone too (for example at the vet). She would howl and do physical damage to the crate if left completely alone (the same crate that she will happily run into if her buddy is home).

When she was my only dog I used to do the kong thing. If you freeze it or alternatively melt cheese in the microwave in it, it will last longer. I believe there are some good "recipes" on the kong website. I also used to put kibble in a buster cube to occupy her. Just don't let them use a buster cube in a room with floor heating vents

She is not very food-motivated but the kong and buster cube were enough to keep her busy and out of trouble.

Oh, and another thing...I found that even when my dogs are together, they get upset if I close the door to the room with the crate when I leave. They would cry the whole time I was gone. If I leave the door open, no problem. Haven't figured out why.