Although Amsterdam is generally safe, nighttime rules about not walking alone in poorly lit and unpopulated areas apply especially to women. Harassment incidents do occur, and rape is not unheard of. It's safe for groups of women to go around the famed (and notorious) Red Light District -- 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 be mistaken for a "working girl." Public transportation is usually busy even late at night, so you generally won't have to worry about being alone in a tram or Metro train. If you do feel nervous, sit close to the driver, if possible. Many local women go around by bicycle at night.
The Netherlands has long enjoyed what one might call a relaxed attitude to the undraped female form -- a government-issued DVD, part of a mandatory "education" for would-be migrants, portrays going topless at the beach as an integral part of Dutch culture. But if close personal observation is anything to go by, I can assure you that far fewer women are actually going without at the beach or in the park these days, and those who do aren't likely to be young.
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.