Drenthe province (mostly)
The carefully arranged piles of massive Scandinavian boulders, known as “giants’ beds” or dolmens, are the most impressive evidence of megalithic culture in the Netherlands (meaning: these rock stacks have been standing around for 5,000+ years). The arrangement sort of creates a cave beneath (which you can totally crawl around inside) that was probably used as a burial chamber. Don’t worry, any bodies are long gone -- except those of the mostly domestic tourists playing hide and seek with their families. And you.
You’ll find the most impressive one in NL in Borgen, next to the archeology museum dedicated to the people who made them (and lots of other stuff, it turns out).