In general it is understood to be illegal to 'wild camp' in the Netherlands, there are however a very small number of places, most often private land, where it may be permissable to stop overnight provided permission is asked.
The only one which I have personally located is at Schoorl-bei-Bergen, in North Holland, where the restaurant 'De Koperen Lantaarn ' is said to allow this. The owner appears to have a motorhome but when I visited the restaurant was closed for holidays and I was therefore unable to stop here. If you want to find it, the restaurant is at the beginning of the village (coming from Alkmaar) on the right at a fork in the road and before the one-way system. This is an attractive area of sand dunes and forest, it is very popular with the Dutch for holidays and weekends. Schoorl has several good restaurants offering 'steak-house' type menus. Alkmaar itself is a delightful town with beautifully-kept houses along the canals, but I found parking difficult when I visited. 
For a place to park for a few hours during the day while waiting for a ferry at Europoort, I would recommend:
Brielle in South Holland, which is well out of Rotterdam on the N218 and close to the Ferry Port. Watch out for SP beyond the end of the motorway out of Rotterdam; turn off and over a suspension bridge, then right at the traffic lights. Allow enough time to reach the ferry port as this area can get seriously traffic-clogged at 'rush hour'. Without a hold-up 20 minutes should be sufficient. Brielle is a very old fortified town, with attractive streets and pleasant canals to walk along. You can park on the parking area outside the walls, just off the road from Rotterdam and about opposite a fuel station. On the right just inside the walls is situated 'Het Melkmeisje' at Kaaistraat 23 ('the milkmaid'), a typical Dutch 'eethuis' with a friendly welcoming owner: reasonable beer, especially Heineken 'Bokbier' when it is in season, and good food from a short menu. The ship models and other nautical paraphenalia give a lot of interest. 
Further in towards the centre on the town square opposite the Tourist Office is 'De Hoofdwacht' at Markt 7, with good standard Dutch fare. To the right down the main shopping street which crosses the square is an excellent cheese shop cum deli, with a far wider range of Dutch cheeses than usually reach England. If you think Edam and Gouda are boring this shop could persuade you otherwise.
Two other 'eethuis' establishments seem worth checking in Brielle: 'Di Sivarter Katz' and 'Kont van Het Paard', sadly neither has been open when I have visited. The first mentioned is on a corner beside the main canal running through Brielle (at the end of the main street which exits right the main square), the latter is near to the main town square (ahead and bear right). Neither appears to open at lunchtime and both have interesting looking menus.
Brielle does have a campsite (not visited) which is some way out up the canal towards the sea.