Erin, is this maybe your first GSD? From your post, I'm gathering that you might be learning all that you can online.
One of the most important tips that I give new owners who adopt from our breed rescue is that exercise is at its best when it's structured -- going for a nice run together, or a hike with hills, or at least a very long walk every day (or even twice a day). They bond when they're doing this. Nothing means more to them than going out and seeing the world together, side by side. There's no electronic device that can duplicate what you get together out of that simple, time-honored activity -- put your phone in your pocket on DND to let yourself be 100% present in the moment, and focus on your dog, the communication, the body language....it's literally the best part of my day (and my dog's day).
If you need "at home" exercise, teach fetch with two balls so you can throw one the second the dog returns with the other, and keep a fast pace going. Others have mentioned flirt poles in other posts.
If you want mental engagement, practice obedience or "find it" games, or take a nose work class. We build obedience "games" into our life so that we're always practicing something or other in a game-like fashion. Dogwise Publishing has a wonderful catalog of books (and e-books) on training "games."
If you can take a good class with your dog and learn some techniques that suit your dog (whether clicker, treats, or some other method), tricks and other things even become possible. Or if you want something more athletic, an agility class is mentally engaging and physically demanding and gets you around people who are really into their dogs and often know a ton (because they've already made all the mistakes years ago and made their way to the other side).
All of this is just to say...you can do so much more for this dog to engage without that device and avoid all of the risk of causing crazed, obsessive shadow/light chasing.