Maastricht, in the south of the Netherlands, is known as the sunniest city in Holland. It's a hub for travel to the countries that surround it: Belgium, France and Germany--but it's also a great place to visit by itself, with caves exploited since prehistoric times, the former Kruisheren cloister dating from the 15th century (now a hotel) and lots of good food and beer, especially around the central Vrijthof square (Het Vrijthof).
Maastricht has a moderate climate, with more rain in winter than the rest of the year except in July.
Maastricht weather is best May through September, although you'll find decent weather in the shoulder season.
The charts below show the historic high and low temperatures and average monthly rainfall for Maastricht.