Kerala is known for its warm and pleasant climate. The coasts and beaches are hot and humid. Light cottonwear and sunglasses are recommended. However, the hill stations like Munnar and Thekkady are rather cold, so one may want to carry a jacket. The people of Kerala are a tad bit orthodox, so one must be cautious with their clothing, especially at the beaches.
Laundry is easily available, so one need not over pack.
Keep in mind that the hill stations are a part of the Western Ghats range. Carry all necessary medication and anti-allergic pills etc. Medication is available in case of emergency, but you may not find the brand you are looking for. Mosquito repellents are an absolute necessity here.