How flexible is your six year old? Cities can be tiresome to them (and their parents in these situations). Travelling through countryside (i.e.southern England or Provence) would entail lots of hours in a car.

Greece is a good option as is Italy, In my travels, I have always noted how much Italians and Greeks enjoy children and go the extra step to accommodate them. You could rent a house in Tuscany or Umbria. You'd have a base of operations which may be important to your son and your peace of mind.

Not sure about the weather in Greece in April, but our experience in Tuscany/Umbria was very comfortable.

A Mediterranean cruise would be another option - a good mix of relaxation and activity.

Lucy, Toronto