We stayed in Arillas years ago, it was very quiet and even less developed than it is now. We quickly exhausted the range of restaurants and got bored of it, but that was about 2002 and it's a bit better now.
However we LOVE Kassiopi, have stayed there many times. We hire a car from the apartment owners (they also do airport transfers which makes it even easier, as their vehicle can carry our luggage better than a little hire car!) so we can take ourselves to Aqualand, Corfu Town, the Donkey sanctuary, Paleokastritsa, Perithia, Kalami beach, St Spiridon beach with pedalos, Sidari...
There are beaches on either side of the harbour, but they are pebbly. This means they are great for snorkelling as the water is so clear. We like to hire a boat in the harbour and go along the coast, stopping for lunch in tavernas along the way. We've also booked boat trips to Paxos and Antipaxos (in pre-covid times) and an evening boat to Corfu Town, which is magical by night. The restaurants in Kassiopi are fantastic, we have stayed there for three weeks at a time and not tired of them. We always go self catering so have breakfast in our apartment, make a picnic lunch or go out for the day and have lunch out. Dinner always in the restaurants.
They did have a problem with rubbish (as the EU imposed laws about landfill which aren't appropriate for everywhere in Europe) but we were there last year and they seem to have found a solution. There are less mosquitoes there than other parts of Greece, we've never had a problem with them.
We are so looking forward to going again this year.