How can teachers keep students motivated in long online classes?

I am a new teacher and still learning how to keep students active in online sessions. Sometimes after 20–30 minutes they start losing focus and look tired. I try to ask questions and give small tasks, but it is still not working very well. Can anyone please share what simple methods or activities you use to make long online classes more interesting and motivating for students? I really want to improve my teaching and keep them engaged. Thank you in advance for any ideas.

This question has been lightly edited by Edutopia staff for clarity.