thamesvalleyman
Striker
The costal / beach bit near den hag is quite nice
Heerenveen is also quite nice, up in the north
Spent quite a bit of time there some years ago. Good place to visit. Groningen also good to see.
Honourable mentions to Utrecht, Volendam, Haarlem and Schevenigen.
If you're already in Rotterdam, you can spend the day at Kinderdijk windmills (250 years old , plenty of walking and cycling around there if you like that).