PM me with anything Netherlands-related. I've also gone to Paris, Milan, London, Prague, Cracow, Berlin and Stockholm though others probably can answer related specific questions better than me.
In general: gas will be much more expensive than in the US, don't let that surprise you. Trains are pretty good in general, I'd say. I'd recommend visiting London, learn to use the underground there.
Funny, I'm planning a generic backpack through the States trip this summer and I could use some help there.
Are rental cars really that expensive? 