Well, I suppose the Netherlands is kind of famous for its constant battles with the water because it is located so low (hence the name), so you might want to see the Deltawerken in Zeeland or perhaps cross the Afsluitdijk (OK, that’s not particularly exciting). You’ll probably find the mills if you explore some smaller towns.
You might also want to see The Hague where the government is seated, or perhaps the canals in Amsterdam (and Amsterdam in general). There’s a lot to see in most of the big cities, anyway, such as Rotterdam and Utrecht.
Furthermore, I’m quite fond of cycling over the dykes. I’m not sure what Hungary is like, but the Netherlands is flat, which makes for a nice view (though many Dutch people tend to disagree).
And of course, if you like museums (I don’t, so I’m just summing up some I’ve heard of), there have been some renowned painters from the Netherlands (Van Gogh, Rembrandt, so you could see the Rijksmuseum or some of the other museums.
You’ll especially want to look for the VVV, the Tourist Board of the Netherlands. Most cities have a VVV building which can help you find interesting sights to visit which are recognisable by the logo.