Rotterdam! there are tons of spots really close to eachother next to the Maas river.
Some of them are pretty famous, Nike sb, Emerica, ZY and and bunch of other companies come by every year to skate those spots.
The best thing to do is go to Left skateshop over there, and they ll make you a spotmap. And most of them are skateable during the entire day!
Utrecht doesnt have that many spots to offer, just 2 or 3 decent ones besides the orange bowl.
Amsterdam is really good for street skating as well but some of the spots are pretty far away from eachother.
Brussel is really good, and then finally Luxembourg has many many spots to offer, go there in the weekend you ll never get kicked out its great!
And its not worth to skate parks, parks are everywhere, better hit them street spot when you got the chance!