West costal regions are safe to travel during mansoon season like varkala, Alleppey , kasargod, kochi, kannur. Please skip western ghats in this particular time like munnar, vagamon etc.
If you are planning to kerala in monsoon so you can visit these places these places are consider safe places - Munnar, Alleppey, Wayanad, Thekkady. so also i have some more places you should avoid in monsoon season - Idukki, High-altitude treks, Beaches during rough seas (like Kovalam or Varkala).