Mountains in Holland? Are you kidding? Well yes and no. The general opinion is that Holland is completely flat. This is not really true. We have stuff we call "berg", which means "mountain" in English. The height of Holland ranges from -6m up tot 321m (m = meter = app. 3ft) above sea level.
The lowest point seems to be the Alexander polder, near Rotterdam. It is over 6 meter below sea level. Not a very pleasant thought, if it would ever happen that the dikes break again, like it happened in 1953. Even in the flat areas in the west and north of the country you may notice height differences. Near the coast, you find the dunes, which are natural sand hills, created by wind coming from the sea. The may reach a height of up to 60 meters.
In the polder land you will see dikes of course. In the North of the country, people created artificial hills ("terpen") with a height 5-8m on which the built their villages. Dikes, if they existed, were not always reliable. These artificial heights were intended to give some protection. You still can see farm houses in the river delta, using this concept.
Ice age hills
During the ice age Holland was at the end of the large glaciers, bringing stones and sand from the North. These sand and stone masses created hills at several places in the Netherlands. For instance the Drentse Heuvelrug in the east of the province Drente. Another example is the Utrecht Hill Ridge, in the province Utrecht with hills up to 60 m. This, you cannot call "flat" any more.
The south east of Holland (Limburg) has hills that were formed by earth quakes many years ago. Later rivers created sometimes steep valleys. The highest point is the Vaalserberg, which is exactly on the spot where the frontiers of Holland, Belgium and Germany meet. Its altitude is 321 m. Every Dutch will visit this spot once in his life. It is not very interesting, but you will find a restaurant there....
Influence of rivers
The Dutch rivers have to find their way through the hills, making sometimes deep valleys. A nice example is the Wageningse berg, formed by Ice, but carved away at the south side by the river Rhine. You can visit this spot by hiking or bike. Hiking you can walk over the hill and have a nice view on the the Rhine. By bike you may cycle through the Rhine valley, just at the the foot of the Wageningse berg.
The most beautiful hills you will find in Limburg. Both for hikers and bikers its really nice. If you are lazy, you may book a tour by train on the "Miljoenenlijn" (Million guilders line). Its got is name because of the huge investment it required to build the railway line. It is closed now, but there are touristic rides by steam train. There is no website in English at the moment, best is to check with the tourist offices when there are rides.