1. Beach
2. Tiger
3. Apple
4. Dance
5. Lemon
6. Cloud
7. Smile
8. Heart
9. Music
10. Angel
11. Light
12. Magic
13. Ocean
14. Plant
15. Quiet
16. Brave
17. Fairy
18. Sweet
19. Water
20. Horse
Remember to randomize the order of the letters and use different fonts to make it even more challenging for bots to decipher. You can also add some background noise or distort the letters slightly to improve security. Additionally, consider implementing a time limit for solving the CAPTCHA to prevent bots from trying multiple times.