Few of my favorites would be
1. Mangrove boating at poovar
2. trekking at ponmudi
3. sunrise of Kanyakumari (reach there by atleast 4-5am)
4. lighthouse of Kanyakumari. you can see the whole Kanyakumari and its breathtaking ( we went there by 9am and they were not yet open but the care taker seeing that we girls had come from afar let us in)
5. Alleppey. you can book a boat house there