There are many fun things you can do with your dog on a rainy day indoors. Here are some ideas:

1. Teach your dog a new trick: Spend some quality time with your dog and teach them a new trick or command. This can be anything from "roll over" to "fetch" to "shake."

2. Play hide-and-seek: Hide a treat or a favorite toy somewhere in the house and encourage your dog to find it. This can be a fun game for both of you and will also help your dog improve their scent-tracking abilities.

3. Make some DIY toys: Create some homemade toys for your dog using items you already have around the house. For example, you can make a tug toy by knotting an old t-shirt or create a treat dispenser out of a plastic bottle.

4. Have a movie marathon: Snuggle up on the couch with your dog and watch some of your favorite movies together. Choose movies with dogs as the main characters for an extra fun experience.

5. Play indoor fetch: If you have enough space in your house, you can play a game of fetch with your dog indoors. Just make sure to clear any breakable items out of the way first!

6. Do some indoor training: Work on obedience training or agility training with your dog. Set up some obstacles like chairs or blankets to create a mini agility course.

7. Have a spa day: Give your dog a bath and groom them, trim their nails, and brush their coat. This can be a relaxing bonding experience for both of you.

Remember, spending time with your dog is always fun no matter the weather outside!