For useful sites, I'd go for http://en.routeplanner.fietsersbond.nl/ . If you click on the tab 'Lf en Knoppunten' you get to see the long distance LF routes (landelijk fietsroutjes/ national bike routes) and all of the waypoints. In theory you can just plan your route the night before and write down all the numbers of the waypoints you need, then follow the signs (red for short distance paths, green for LF routes) to the waypoints. I've managed to get lost doing this before though, so you might want to keep a map too. At each waypoint (knoppunt) there's a board with a map showing nearby waypoints.
You may feel like after Ijmuiden to Gouda to Utrecht you fancy some slightly more undulating countryside, in which case you should head east from Utrecht, perhaps towards Arnhem and Maastricht. To the west of Utrecht it is completely flat, to the east there are occasional small hills.