I second the board game recommendation (my personal favorite is the ancient game of Go). Playing a musical instrument is another great option. If you don’t own one, you can play around with a virtual keyboard on your computer, for example. There’s plenty of indoor hobbies to take up, e.g., sewing, drawing, reading, cubing, etc. If you are able to go to another place indoors, a gym is a productive option. My preference would be a climbing gym because it’s fun exercise.