My 5 tips:
The most beautiful building in the world
The Kunstmuseum Den Haag by the famous architect Berlage. A wonderful example of Dutch architecture and a great art collection.
This has to be a definite must
Walking in the dunes "on" Scheveningen or at Kijkduin. Or even better: to go running.
Should not be missed
Something not to be missed: the parliament buildings and surroundings. Take a picture of the most beautiful skyline of The Hague from the Lange Vijverberg, with the Hofvijver in front of the photo.
The best food
The Denneweg is a world famous street in The Hague for going out for a meal. Highly recommended. I love Indonesian food myself, with the Soeboer restaurant on Brouwersgracht as my favourite.
Secret tip
The Papaverhof from 'De Stijl' architect Jan Wils from the 1920's in the Bloemenbuurt. Still beautiful and modern, but also intimate and tranquil.