The Treadmill Stroll
A treadmill is a great way to get your dog a dose of healthy indoor exercise. First, allow your dog to get comfortable with the sight and sound of a running treadmill. Next, place your dog on the treadmill and give him a treat. Turn the treadmill on the lowest speed. Give him treats to keep him on the treadmill. You may use the dog’s leash as an aid but never tie your dog to the treadmill. Also, standing in front of the treadmill and rewarding him with treats for walking might make your dog feel more comfortable. Once your dog is adjusted, you can gradually increase the speed to provide a more challenging workout.
The Stair Game
Using the stairs is a great way to tire out your dog. The steps add an additional challenge to a dog’s workout, as they engage different muscles from those used on a regular walk or run and add an extra level of difficulty with the change in elevation. Stand at the top of the stairs and throw a toy down to your dog. When your dog grabs the toy, call his name and have him bring the toy to you. After several rounds of this, he’ll be winded.
Or run up and down the stairs with your dog to get a workout in yourself!
Italicized excerpts from: https://www.cesarsway.com/how-to-exercise-your-dog-indoors/