Definitely go on this trip it's a great experience.
Don't expect to be able to go to lots of bars, there isn't the same kind of drinking culture in Kerala as there is at home, but Kingfisher beer is available in most places!
The day walk from Mattupetty Dam was a lot harder than many people anticipated. The walk is really quite steep all of the way up and combined with the humidity / heat does make it quite difficult. That said it is worth doing, the view from the top is great and it was good to do something more active.
The air con on the house boat was really cold at night, take layers!
I wouldn't recommend the visit to Kanyakumari - we spent a lot of time queuing and there really wasn't anything much to see. The wooden palace about an hour away was beautiful so perhaps that could be a shorter optional trip.
The German bakery, Waves, at Kovalam is a must visit restaurant - food and cake is fantastic, as is the sister Fusion restaurant.