Kodachadri Nilgiris - Ooty Refer my blog on Tripoto Queen of Hill station
Also Kudremukh is amazing - I'm yet to write blog about it. Feel free to ping me if you need any info
I don't know which place in South India you would like to visit.. But I'll tell you some of the best treks in Karnataka.. 1. Kanoor fort trek - 3 days
2. Kodachadri trek - 2 days
3. Kumara Parvata trek - 2 days
4. Mullayanagiri trek - 2 days And still many are there.. All of these are at a over night journey distance from Bengaluru..