Belgium, ehm, isn't that the stretch of land we Dutchies pass when we go to France? :angel:
As Roger mentioned, don't do to much. And don't travel too much, somebody mentioned the South of France, however, there is a lot in between Belgium and the South of France that you will be missing. Belgium is full of history, Brugge is a must do location as is Antwerpen and Gent. The Ardennes is not only famous for the Ardennes offensive (aka Battle of the Bulge) but also for its beautiful hills and woods, the city of Bastogne (Bastenaken) is a good starting point to visit the Ardennes.
In The Netherlands try to visit the Delta works, it is a magnificent piece of engineering in Zeeland province. Another place to visit in the Netherlands are the Bieschbosch, a unique nature reserve. Also do a daytrip to Den Bosch and make boattrip on the Benedendieze, it is a very nice experience.
If you're a WWII buff, you may consider the places connected to operation Market Garden near Eindhoven, Nijmegen and most notably Arnhem. However, Arnhem might be too far.
In Belgium, make sure you visit the city of Ieper (Ypres), it was completely rebuild after it was destroyed in WWI and that is an interesting experience even if you're not into WWI. Canadian troops fought during the second and third battle of Ieper. There are several other interesting WWI sites in Belgium and the North of France if you're interested.
Waterloo is in Belgium, so visit that. The Belgian coast really is nice, but the water will be cold in April. Visit Luxembourg, it is beautiful.
Some more advise. Belgium is bilingual (actual trilingual), however, a lot of Flemish folk don't want to speak French and a lot of Walloons don't want to speak Dutch. Just be aware of it.
Dutchies usually speak well enough English for a conversation, the French usually not. The Belgians are in between.