Is it better to wait and buy tour tickets (canal cruise, windmill tour) at train station, hotel, or in advance (VIATOR etc.)?
Did you realize Viator is a 3rd party consolidator owned by Trip Advisor? They charge 20-25% to the vendor for the listing so it's much kinder to book directly thru the actual company running the tour. I'd not book a canal tour until the actual day as you'd probably want it in good weather. I've not done a windmill tour.
I would wait and buy them once I'm there. They will not sell out (unlike Anne Frank's house, etc.)