Ten days gives you the whole of Kerala in one line: the colonial streets of Fort Kochi, the tea hills of Munnar, the forest and spice country around Thekkady, the backwaters at Alleppey and then the beaches at Varkala and Kovalam. It ends in Trivandrum, so there is no backtracking.
Each drive is under about five hours, with two nights in each of Munnar, Varkala and Kovalam to slow the pace. Varkala is a cliff-top beach with a relaxed, backpacker-meets-yoga feel. Kovalam is more resort-like.