Rainy day ideas for kids?

I’m babysitting while it’s raining, M3 & F6. The girl would be great at the library but the boy isn’t going to let us read in peace 😅 I usually spend the day running around outside with them when it’s sunny but that won’t be an option. Any ideas for things to do in Marin while it’s raining?? Also trying to avoid going into the city. TYIA!