I'm really sorry to hear that Annie has a torn cruciate!
You might try some scent exercises. Initially you could just teach the "which hand has the treats" game where you have food in one hand and hold out both to her to figure out where the treats are. With my dogs I just ask them to nuzzle the right hand. This could be done with her in a down position as long as you keep your hands low.
And then you could teach her to find a treat in the room that you've hidden, if she's able to walk around a bit. You start out simple and gradually make it more complicated.
You could also teach her a structured retrieve. I posted some information on the basics in a thread just today - here's the link: https://www.germanshepherds.com/forum...e=1#Post923290
For her, you'll have to adapt it to her laying down, but I think it could be done especially if you sit on the floor with her. The actual training involves her looking at the dumbbell, and when you get to the point where she needs to touch it you can just be sure to have it within range where she barely needs to stretch to reach it.
Also in that same area is a training discussion on how to teach dogs to put their heads into the collar, which can be adapted to fit your dog in whatever way you might want. The link is here: https://www.germanshepherds.com/forum...e=1#Post920556
It's so hard keeping an active dog confined. I know with Tazer (who is healthy but has to be confined at times because she is a destroyer) it has really helped to have chew bones and other things that engage her mouth.
Good luck, hope she heals quickly!
Melanie and the gang in Alaska