I've organised these events a few times and they've always been a great success. (check out the last event we did HERE)
In Holland there are NO naturally occurring rocks - however there are world respected climbing walls; Four in Amsterdam alone.
The climbing wall at Sloterdijk (Klimhal) has a 21m main wall for the brave and smaller walls for those of us not so brave which are perfect to begin on.
Check out the location on google maps to avoid getting lost
They offer an “Introductory lesson”, where you learn safety procedures and receive expert instruction on how to climb.
This lesson including entrance and all the safety equipment only costs 19.50 euro. You won't need to bring anything except a sense of adventure
If you've never tried this is a great opportunity to give it a go in total safety with professional supervision, invite friends / family / loved ones / neighbors
What to wear?
You can climb in pretty much anything BUT I would recommend wearing clothes which allow freedom of movement, shorts, sports trousers kind of thing and a T-shirt as it can get quite warm.
Strong footwear like running shoes are fine (though they do need to be clean), alternatively for a small fee you can rent real climbing shoes which are better.
Once the lesson is finished you will be free to climb in groups to your hearts content or sit and enjoy a well deserved beer and chat.
If you're already a climber, why not come along anyway and just pay the GYM entrance fee. (Let me know you'll not need a lesson)
I'm part of a climbing community, we're a friendly bunch check us out http://amsterdamclimbers.com
Hope to see you there.