Avalon Waterways have some great offers on five river cruises on their “Special Offers” page on their site. I particularly like this Charms of Holland and Belgium – Brussels to Amsterdam cruise with departures from June to September.
“Explore the tranquil beauty of the Belgian and Dutch waterways with Avalon Cruises on this fascinating river cruise from Vilvoorde to Amsterdam. Cruise across the heart of Belgium and into Holland, taking in the charming towns of Middelburg and Willemstad as well as the cosmopolitan cities Antwerp and Ghent. Included excursions introduce you to the fascinating Delta Works near Veere, traditional Dutch windmills in Kinderdijk, and either the glorious Keukenhof”