The Ethnographic museum of Leiden (not to far from Amsterdam) has an excellent collection but little on display.
they will have however have a great exhibition on Sumatra with many museum pieces from all over the world. I will open on the 14th of october.
I am looking forward to going there!
http://www.rmv.nl/index.aspx?lang=en
Bronbeek is being renovated so I would not go there although it is my favorite museum in the Netherlands! During the renovation only one floor is open and it is quite a trip from Amsterdam.
Regards, Erik