I lived in Holland for 10 years lived less than 10 miles from Duinrell. Great fun for the children
young & old, if you have time visit Haarlem, Amsterdam, & Leiden. Amsterdam is still my favourite city in the world.
Lived in Rotterdam for a while. love the place, love the people.
Take some bikes, or hire them anywhere, and go for an explore of the bike paths...... use the trains too.. try the Indonisian food, order Crokettes (unidentifiable meat in a breadcrumb coating) at every opportunity you can, have apple pie and cream with a coffee.
If you go to Amsterdam do go to the Anne Frank house. Also, Delft is a lovely place to visit. Its where the Lady with The Pearl Earing was set in real life, home of Vermeer. Proper little canals and squares and Windmill by the station. Looks like thats within easy cycle of where you are staying. If you end up in Rotterdam do the boat tour of the docks and have beer in the Old Haven by the funny cube Houses.
try and visit the flower auction, either the one near Rotterdam or the largest one at Alsemeer.