I live 15 km from Valkenburg and the riding is really good, don't think the Nederlands is flat, it is not, bit like saying England is flat because you went to Norfolk once. The links above give you some good idea, I can't seem to add a link at the moment. If you have Strava or Garmin Connect, you can find lots of routes in the area.
The MTB routes are generally well marked with a small sign with a black triangle and 2 black circles below. Signs are various colours depending on the route.
Maastricht is well worth a visit, lovely historic city with plenty of bars/restaurants/cafes. Not too much in Valkenburg and it will get rammed with fans over the weekend, personally I'd either pick a spot to watch the race and stay there (arriving early), or go out on the route to spectate, again though it will be busy. Not too impressed with Liege, get the impression it will be nice when they finish it, and they have been working on it for years.
Most cyclists are "Lycra'd up large" they don't seem to mind being a rolling advertising hoarding, they are also very reluctant to say hello when they are out, suppose it is because there are so many of them.
Unfortunately I'm working night shifts over the weekend, otherwise I'd happily meet up and show you round. The are is fantastic for cycling, both road and MTB, especially if the weather is good! If you want to know anything else, give me a shout, just sorry that I can't watch it this year, does mean I can do Liege-Bastogne-Liege though!