There are many good indoor activities that you can do with a German Shepherd Dog. Here are a few ideas:

1. Training: Training is a great way to keep your dog mentally and physically stimulated. You can teach your dog new tricks or work on obedience training.
2. Puzzle toys: Puzzle toys are a great way to keep your dog’s brain active. There are many different types of puzzle toys available, such as treat balls and Kong toys, that will challenge your dog to figure out how to get the treats inside.
3. Play fetch: Playing fetch is a classic game that many dogs love. You can play fetch inside with a soft toy or ball.
4. Go for a walk: Even if you can’t go outside, you can still go for a walk in your home or apartment. Encourage your dog to walk with you, or try using a treadmill for dogs.
5. Grooming: Grooming your dog can be a bonding experience and it’s also a great way to keep your dog looking and feeling his best. You can brush your dog’s coat, trim his nails, and clean his ears.
6. Dog sports: There are many dog sports that you can participate in with your German Shepherd, such as agility, obedience, and rally. These sports are a great way to keep your dog active and engaged.