Most basically, the weather is more pleasant from the months of November to the mid of February in Kerala. Due to the higher flow of tourists then, almost all the hotels and tourist centres would be crowded. So you may book or reserve everything prior to your visit. However, to experience the monsoon in the state, you may have to visit from June to September.
Try not to include waterfalls and picnics at the riversides from your bucket list visits if you had planned your journey in the summer season, as they will be dried up and may look unattractive.
Ensure that you are carrying enough insect repellent before setting out in the sun and also at night.
If you are wishing to witness the real celebration mood of the state during the festival of Onam, you may book your tickets to the last week of August or towards the beginning of September.
It would be wise to book your air tickets to Cochin International Airport, once you had included Munnar, Allepey, Thekkady and Kumarakom in your travel itinerary to Kerala.