Amsterdam is Europe's ideal singles city: Countless singles from around the world come there to have a great time. The other major Benelux cities, not so much (Antwerp and Rotterdam could be the big exceptions). Amsterdam is not that difficult of a place to get to and get to grips with for a person traveling alone. There are lots of hotels and restaurants in all price bands, and it's easy to meet other people around the bars and clubs, on canal-boat tours and bicycle tours, and in other ways.
I don't know if anybody ever went to Brussels, Bruges, or Ghent for a no-holds-barred wild-party vacation -- and for sure not to the Hague, Liège, or Luxembourg City. I don't mean to say there are no decent bars and dance clubs amid the fine dining, art galleries, history museums, and other sober-sided goings-on in these places, but that's not really where their heart and soul are.
Companies that specialize in solo travel to the Benelux lands are thin on the ground. The Singles Travel Company, 56 N. Santa Cruz Ave., Los Gatos, CA 95030 (tel. 888/286-8687, or 408/354-3871; www.singlestravelcompany.com), takes in Amsterdam as a part of its European tour.
Note: This information was accurate when it was published, but can change without notice. Please be sure to confirm all rates and details directly with the companies in question before planning your trip.