Wow does your holiday really start in May? Lucky you!
Here summer holidays usually don't start before the end of June.
I agree with the rest, you should definitely visit Paris, Rome, Florence. Naples is also cool, then you can climb the volcano
Also nice: Lisbon, and I heard Prague is very beautiful.
If you decide to stop by the Netherlands too, don't forget to let me, Yshka and the other Dutchies know