In Amsterdam, it's safe for groups of women to go around in the city's famed (or notorious) Red Light District -- always supposing they can stomach seeing other women serving purely as sex objects -- but a young woman on her own, particularly after dark, could be subject to at least verbal harassment, and misrepresentation as a "working girl." All other red light zones in Benelux cities -- and especially those in Brussels and Antwerp -- are best avoided by women.
Public transportation in most Benelux towns and cities is usually busy even late at night, so you generally won't have to worry about being alone in a bus, tram, or Metro train. If you feel nervous, sit close to the driver whenever possible.
Holland has long enjoyed a relaxed attitude to exposing nontrivial amounts of the undraped female form -- a recent government DVD, part of a now-mandatory "education" for would-be migrants, portrays going topless at the beach as an integral part of Dutch culture. Far fewer women are actually going without at the beach or in the park these days, and those who do are less likely to be younger women and teens. Catholic Belgium and Luxembourg always were less relaxed about this, and remain so.
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.