Hi t_maia,
Free camping is officially not allowed in the Netherlands. Since a while though, our national forest conservation has a list of free camping places mostly in or at the edge of forests. You can contact hem on their
website to ask if they have information about that in English (website is rather limited in english).
For more information about campings in the netherlands check this (bigger campings) or this (smaller green campings) website.
There is a national cycling organisation in the Netherlands but their website is in Dutch only. You might contact them as well for routes, because there are good biking routes in the Netherlands (green and red signs) which are generally in the more rural areas as well.
As of highlights apart from cities, I would certainly try to visit the northern provinces (Friesland, Groningen and Drenthe) as well as the southwest province of Zeeland, which are all extremely quite compared to more west and central parts of the country. Also, there are some nice national parks in this region. This applies to Overijssel and parts of Gelderland (Veluwe is nice) as well.
In the extreme southeast corner of Zeeland you can find the Drowned country Saefthinge, which looks a bit similar like Wadlopen, but the circumstances are more different.
Be sure not to miss the massive and impressive Deltaworks in this province. The link provides more info about activities and places in Zeeland as well, like another nature walk at Zwin, on the border with Belgium (extreme southwestern point of the Netherlands). You can cross by boat from Vlissingen to Breskens (bikes, walkers only).
The disadvantage is that Zeeland is just a bit far away, but you can always take a 2 hour trainride or so and take your bike.
Have fun, hope his helps a little.
Cheers!
Michael.
[ Edit: Edited on 05-May-2009, at 07:44 by Utrecht ]