You should go for hilly areas as they are colder in these times other places are more hoter and it will be a mess to go in cities that are hotter you shoul prefer the places such as shimla , manali and something like that
You can go to Agumbe in Karnataka, it's known for its amazing rainforest, waterfall and wildlife. Agumbe has the most rains in the South.
If you are a fan of Malgudi days, it's an added bonus because it was shot in Agumbe.
Do try their amazing coffee and enjoy the rains
This is a bit difficult to answer! Going by the size and diversity of India we almost have everything starting from the hills to the sea to mangroves to the desert. So it completely depends upon one's own taste. Like I personally like the beaches.Goa, Pondicherry,Gokarna in Karnataka are some of my fav places.
If you wanna explore, get some mountaineering skills and head towards himalayas. Mountains bring out the Explorer within you. They show you the real reason of existence of nature. Try to climb all the mountains and explore the wonderful scenery. Only if you climb a mountain, you will know the happiness of saying 'I made it'.