Hi, I would second Munnar. The climate is cool & freshing compared to the humid coastal areas. It's really beautiful travelling through the tea plantations and cardamom hills. I also stopped at a place called Kumily, which is well known for herb gardens and the Periyar National Park, where you can do some easy treks.
I also spent some time in Cochin, which is a fort town with some interesting history. And of course what gocebe suggested, there is nothing more relaxing then taking a trip down the back waters. I only did a day trip as I was on my own, but if you're with a companion I'd recommend doing an other night trip.
I only spent 6 days in Kerela but managed to cover all this. I had been working in Bangalore & flew down to Kerela. As it is a small state you can cover quite a bit.
Have a fab time, I loved it. 
[ Edit: Edited on Nov 26, 2008, at 2:28 PM by Laura_B ]