You can take the konkan route and cover Goa, Gokarna, Mangalore, Wayanad, Kannur, Munnar, Vagamon, Alleppy and fly or take a train back from Kochi. This is the best route to cover most of the beautiful places in South India in less days
If you are looking for Kerala, u would definitely suggest fort kochi and munnar. Kochi is known as the queen if Arabian sea and munnar is having the lowest ever temperature. so I would suggest these.
other places I would suggest r ooty, coorg, Danushkoodi, nandhi hills.