These are intended for elementary-aged students, but plenty can be adapted for older kids (and open-minded grown-ups) as well!
1. Use the building blocks of language.
2. Summon the power of song.
3. Get curmudgeonly.
4. REALLY curmudgeonly.
5. Or rhyme-y.
6. Or conspiracy theory-y.
7. Walk in someone else's shoe.
8. Imagine a terrible, wonderful future.
9. And help beyond your wildest dreams.
10. Use this incredible book as a jumping-off point.
11. React to something in your environment.
12. Induce fear.
13. Lend a hand.
14. Create a new world.
15. (Or a new person.)
16. Report on the world around you.
17. Imagine yourself in new situations.
18. Grade yourself.
19. Make a game of it.
20. Slow things down or speed 'em up.
21. Argue about life's big questions.
Writing Prompts features a ton of rad ideas, and check out Teachers Pay Teachers for a treasure trove of teacher-generated resources.
And if you have any prompts that have worked well for you in the past (either as a student or a teacher), feel free to add them in the comments!