Exploring France
 

I would agree that France is a great option in Europe, especially while traveling in May. Plenty of museums, castles and walking opportunities as well, thus you should have no problem coming up with a suitable itinerary according to your preferences. As for planning the itinerary itself, I suggest spending some hours reading travel guides, marking down the attractive sightseeing options and pointing them out on a map later to come up with a route that makes sense. At least that's the way I usually do it when traveling. Then comes the time to plan transportation, buying tickets, etc. Make sure to check for online tickets to museums and other similar locations, as this way you can often save some money and time (f.e. not having to stand in lines).

Good luck with the planning and enjoy your trip!