You'll probably get better info from others, but... I find Chennai a difficult city to get on with, at least in the short term. There are sights worth seeing (the fort, the cathedral, the beach and lighthouse) and some fantastic food about, but it's one of those cities where you really have to handle the heat and humidity, and know all those little areas to go to for specific needs.

For a short trip, I'd be tempted to head down the coast to Mahabalipuram. Great combination of sea, wonderful ancient monuments, and some really good seafood. But that depends on what kind of stopover you want!