I see a lot of suggestions. here are my 2 cents.
You are going to run for shelter in chennai during summer. You would need lot of water of course.

be careful when you approach the roadside vendors for negotiations
make sure you are not cheated by the auto drivers
dont try all the roadside small snack centres
chennai water would not suit you for sometime.
ok i think this is good enough to scare you.

now good things:
Chennai is one of the best places if you have a friend to show you around who can handle the auto wallahs, road side vendors and chennai traffic.

Lot of places to hang out,temples, churches particularly.
Beach to have a good walk,
Santhome church, Besant Nagar Church to enlighten you spiritually and
Pondy Bazaar, T Nagar for purchases etc… and the list goes on.