Corfu, you dont really need a tour, you can just walk in to the old town, about 20 minutes away from the dock. Katakolon is a sleapy little village with a few shops so personally id go on a tour.
We have been to Katakolon twice and each time we have not gone on a trip and have just looked around the town. We really like it there - its a good place to chill out. There are some nice bars, restaurants and shops. We especially like a little bar which is on the left hand side of the street as you are walking into the town. It is a converted barn and is very quirky. We did this cruise last year along with the Olives & Ouza week. We loved it.