Diamond District, Amsterdam
This may not be much 'off' the beaten path, as Holland is known for its diamond trade, but visit a diamond house - many of them offer tours and demonstrations of polishing & cutting.
Some will require reservations, so be sure to check ahead.
I stopped at amstel diamonds (www.amsteldiamonds.nl) but missed the scheduled time, and didn't make it again - people I spoke to said it was worthwhile, and free, so what the heck!
The diamond stores are something that is unique to Amsterdam.When you are there do not forget to visit one of them.