Teachers often work nights and weekends. They'll be doing anything from helping with extracurriculars, to looking after kids on excursions, to marking and writing reports, to for-profit tutoring, to planning lessons. A science teacher will stay up to date with what's happening in their speciality, and might have semi-related hobbies like birdwatching, or archaeology.

It's a little harder to pin down than side projects, but the activity is still there.