I am Dutch, but have little experience in being a foreign tourist there. I guess one of the islands, as Schiermonnikoog, could be a nice place to visit. Very open, rough beaches ect. Haven't been there myself yet. Some people like to "wadlopen", which is walking to one of the islands from the mainland, over the seabottom that has fallen dry. Do this only with a guide! It is very dangerous to do without guide. E.g.
http://www.wadlopen.com/eng/index.html.
I would concentrate of visiting small towns. I grew up in Schoonhoven, which is a nice town for tourists with silver, clocks and some nice old buildings and a few on a big river. There are several such towns, like Oudewater, Vreeswijk, Volendam. A bigger one I am familiar with is Gouda, with candles, cheese and also many old buildings. From there you could cycle through the "reeuwijkse plassen" area, which is an area full of small lakes with small roads in between. Or through " de vlist", along a river.
I have also heard of tourists that hired a flat boat, "platboot", to sail slowly over quiet rivers.
In the south, in Valkenburg and Maastricht there are caves to visit. In the Lelystad area you can visit a copy of a 17th century VOC ship.
http://www.bataviawerf.nl/voc-ship-batavia.html.
If you like to hike, the "Drunense duinen" is a forest that is a bit rougher than usual and contains some sand dunes.
There are also some old castles to visit, like "Slot Loevenstein" or Muiderslot.
In Belgium, Brugge is a nice smal town to visit.