Ah my spiritual home
Kassiopi has grown and whilst still traditional in parts, is leaning towards the karaoke end (not that I dislike karaoke per se, been known to warble a few). Picturesque certainly but I would say fairly quiet in terms of things to do.
Water sports wise, head to the South West (St. George South) as the sea is choppier that side 
Kassiopi isn't too far a drive from Agni which is beautiful for a day. Has an air of exclusivity about it and Taverna Agni seems to be where celebs/dignitarys/corrupt mp's/oligarths etc dine. Whilst the food is delicious it's well over priced in my opinion and the owner is up his own arse The taverna next along is better imo. Stunning yachts on the right day.
Corfu town is a must. A good wander up and down the alleyways. It's mega hot there though, and again, if you eat on the Liston it's tourists prices. We took a boat trip there and when we stopped for lunch we told the captain we wanted to eat where he was eating. Cue a visit to a small cafe affair which looked nothing on the outside but food to die for.
The water park Aqualand is very popular. 35 euros last I visited so probably about 40-45 now.
We've visited the last 15 years and stay in a darling town called Moraitika (south east) It has a perfect mix for us of plenty of tavernas, few bars, gorgeous old village at the top. We love it 
Hope that helps