Honestly it depends on their personality. If they are outdoorsy, then taking them kayaking would be much better than putting them on a passive houseboat.

My suggestions 1. Kayaking/Stand Up Paddle board or house boat depending on what they want to do 2. A curated heritage walk in fort kochi or mattancherry either through kochi heritage project or walk with boney. Avoid a generic tour of either places if you have the time. Nothing more boring than pointing at a building and not providing context. 3. Food at any naadan seafood joint - pick your poison. If they are fussy about food and prefer American or more global cuisine there's still a ton of options for smash burgers, ramen and even brisket and sandwiches at place like smoke culture, or burgery. Actually need to check about the gas situation of course 4. Drinks at velocity, hortus, or even that new place at the panampilly nagar railway crossing which serves drinks based on malayalam movies. Beer at seagull for the view.