World's your oyster tbf. Rotterdam gives you a lot of scope for locations. Any of the long distance cycle paths will give you a nice journey.
Lots of stop over locations, big range of budgets. Be prepared for some of the locations to expect you to park your bike with the city bikes though, my default is to not ask if I can put my bike in the room and just do it anyway.... quite a few places sometimes don't have lifts and very steep old style european staircases.
That said there is a useful organisation called Vrienden op de Fiets
https://www.vriendenopdefiets.nl/en which is like a warm showers style accomodation based on arriving on foot or by bike, you need to join to get access to the list of hosts and it could be quite an immersive experience.
There's quite a few UNESCO sites for windmills (and a fair few breweries) which are worth ticking off...
Near Amsterdam,
https://en.wikipedia.org/wiki/Zaanse_SchansNear Rotterdam,
https://en.wikipedia.org/wiki/KinderdijkAlso Belgium oppertunities and a host of WW1/WW2 bits too. The enclaves of Baarle make an interesting trip where you can stand eitherside of the NL/B borders in multiple places, or if you venture further in you could even put a bit of your bike in NL/B/DE at
https://en.wikipedia.org/wiki/VaalserbergNational Park Veluwezoom is worth a ride through for the contrast it gives after such a long flat topography, excelent B&B in Velp too (
https://g.page/mtbclinic?share ) which I stayed at a few weeks ago.
Loads of options realy...
I note that I found food quite expensive last time I was over there, a simple tray of frits, frikandel (sausage) and a fanta was 7euro which I thought quite expensive and most meals were 20-30 euros