Sorry Puneet, I have not done any treks in South India. In fact I ain't much of a trekker rather a road tripper with self drive and ride experiences of trans-Himalayas :) ... You should check with the expert trekker and book author Tarun Goel (http://www.tarungoel.in/)
Hey bro, Trekking south india in monsoon is not recommended.
Level depends on your trekking experience, if you are a regular trekker with basic fitness, you can do any of them.
Comparing it with himalayan treks, you wont find the snow, but you can touch the clouds and witness the best of greenery.
Some of the treks in kerela includes Anamudi Peak, chembra peak, ranipuram & agasthyakudam.
Best of the above is chembra peak as per my experience.
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