I found this website that was very helpful
http://www.fitnesstreats.com/2011/02/gy ... -amsterdamThey list a number of options, I was frustrated by the lack of prices on the websites though plus from reading that article it would seem most have a 12 month membership which is no use for people on a short stay.
Some seem to offer a trial month though, may be worth considering. No doubt there will be some pretty strict rules that apply for that though.