RailFanning is an art-form; RailFans are the paints - Ayush Kumar Singh

So,
You can very well use this opportunity to visit Tirupati Devasthanam to visit if you have faith for religious places. While coming or while returning.
another place is Arunachal Mountain and Raman Maharishi Ashram.
Chennai itself is very good place to enjoy, its beaches, and south Indian food,
Next...
more...

is Madurai, Meenaksi Temple and its granduer, along with other temples
south most is Rameswaram, and nearby is KanyaKumari , both are nice place to watch and worship the sea.
I have also heard about a temple with 1000 pillars , but its town name is not sure.
Apart from this place, You have hill stations, Ooty, Kodaikanal.
Many private bus operators/tour operators are available. Just click the Parveentravels.com, and you get huge information, for small trip, long trip, chartered vehcle, hotel booking.
Many more places of interests are there but I have known this much.

Dear Saurav:
In TN which placed you would like to see?, what is your choice of interest beaches, wildlife, hill station etc. TN has has all sorts of places to satisfify the travellers need.
Wildlife: Mudumalai Sanctuary & Parks in Nilgiri Hills, Indira Gandhi National Park of Annamalai hills, Kalakkadu Wild Life in Thirunelveli .
Hill stations: Kodaikanal, ooty, Yercadu, Yelagiri, Kolli...
more...

hiils, Annamalai Hills.
Temples: Madurai , Thanjavur, Rameshwaram, Kanyakumari, Chidambaram, Velankanni, Nagapattanam, Kancheepuram
Beaches: Marina beach, Mamallapuram , Kanyakumari ,
Cuisine: Chettinad, Tamilnadu cusine,
Poompuhar for handicraft. Above given are some of the information one can plan according to their choice.