We are trying to put trip together and would appreciate any possible help:
Day 1 Saturday: arrive in Dusseldorf and get a car. we are planning to spent 3-4 hours there. Not sure if we should spent the whole day. Any advice on a good plan on 'how to see more in short time" and also advice for nice place to have lunch, coffee, beer, ice-cream and where to park the car?
from Dusseldorf we planning to drive to Cologne. it's just happened that we will be there on July 14 - day of the light show. Again we are not sure how big the crowd is going to be and also since all tickets are sold out already if it's worth going without tickets and where to? and basically same questions : where to park the car, have dinner, etc?
Day 2 Sunday: we meeting with our friends in Brussels at about 3 pm. May be we can stop at some nice small city in first part of the day?
Day 2 (afternoon) Brussels.
Day 3 Monday: Brussels.
Day 4Tuesday: Bruges
Day5 Wednesday: ???
Day 6: Thursday: Relocating to Amsterdam and sightseeing Amsterdam
Day7: Friday: ???
Day8 Saturday: Bremen or Hamburg or?
Day 9 Sunday: cruise from Kiel. Drop of the car.
Sunday - Sunday - cruise.

Allow yourself a bit more time in Amsterdam, it's very picturesque and there are a lot of things to see there. But it's probably a good 4 - 5 hrs from Amsterdam to Bremen so you'd need a day for that. You might take the road over the Afsluitdijk (where there is a small museum documenting its construction) and stop in Leeuwarden or Groningen on the way. I visited Kiel about 20 yrs ago and did not find it an attractive city so I would not budget any time for that city.

Lavandula

First suggestion - you will not need a car in Düsseldorf, Köln, Brussels or Amsterdam. Hassling with and paying for parking isn't worth it, in my opinion. And, unless you plan to drive through the countryside and visit small towns, you can do this trip entirely via train and save lots of car headaches.

I would trade one day in Brussels for an extra in Brugge. That can be your "nice small city."

And,I agree with Lavandula, give Amsterdam Wednesday and Friday.

wanderfrau's first suggestion is the best.
You itinerary does not reflect any countryside road trips, but only travels between cities that are well connected by rail. And given the traffic in that part of Europe, the trains will be faster than any car anyway.

IF the car remains a must, you should indeed return it in the country you rented it.
In that case, please forget about the car at least until day 2.
In either case (car from Cologne or no car at all) I would skip Düsseldorf and take the direct trains from DUS airport to Cologne. The few hours you have you can spend there more meaningful.

Can't help with the light show, sorry, never been there for that event. (Not even quite sure what it is)

Pick up car when you leave Cologne.
There are several places along the route to Brussels that are good for a stopover: Aachen, Maastricht, Liège. Though none of those are exactly quaint little towns. More like mid-sized cities already.
One really beautiful "quaint" Flemish town would be Leuven. Almost on the outskirts of Brussels, though. So not sure if that fits your idea of an halfway break.

When day-tripping from Brussels, you will probably start when the rush hour is still at peak. It can take a relevant amount of time to drive in the city and to get out, depending on where you/your friends live. Again, the train would usually be the much faster and nerve-wrecking option, unless your friends lived somewhere on the Western edge of Brussels with easy access to the Ring 0.

There would be a ton of cities, towns, and landscapes to see between Brussels and Amsterdam. But your itinerary makes you just hit the motorway and go point-to-point.

If there were low-budgets connections I'd even suggest to fly Amsterdam-Hamburg as the countryside stays pretty much the same for the whole trip - aside from the adventure to cross the Afsluitdijk.

Hamburg/Bremen. I'd chose Hamburg if I wanted a nice city. But research Luneburg and Lubeck as both are worth a half day visit

thank you everybody for reply!
since we already have a car rented, and all of you mentioned countryside - any suggestion on a specific countrysides would be much appreciated., as well a advice about parking.

I think you itinerary simply does not leave much space for any exploring of the countryside.

Trying to break down your itinerary in the given segments, I could think of the following:

Düsseldorf-Cologne:
not much countryside, take the motorway

Cologne-Brussels:
urban stopovers: the ones I mentioned above
countryside stopover: Nationaal Park Hoge Kempen

Brussels-Brugge:
stopover Gent (though much easier as day trip by train from either Brussels or Brugge)

Brugge-Amsterdam:

The direct route would take you on motorways via Antwerpen, Breda, Utrecht to Amsterdam.

Alternative A, still more focused on motorways:
Antwerpen, Dordrecht (visit, oldest town of NL), Kinderdijk (the famous windmills, visit), few kms more along the river to Bergambacht (via ferry), Gouda (visit, famous stained glass in the cathedral), Amsterdam (by what ever route your GPS says).

An alternative, more time-consuming, would be from Brugge:
Breskens, car ferry to Vlissingen, Middelburg (visit), N57 across the Delta dams and barriers to Rozenburg (west of Rotterdam), car ferry to Maassluis, N468 to Delft (visit, one of the prettiest Dutch/Flemish towns of all), A4 to Leiden (visit), A4 to Amsterdam

As that leg from Amsterdam to Bremen or Hamburg is quite long, I don't know if you have much more options than the stopovers mentioned by lavandula.
Between Amsterdam and Hoorn you could use the scenic drive along the water: Durgerdam, Marken (small pittoresque village, visit), Monnickendam, Edam, Hoorn, then get on A7 and go North.

PARKING:

In most cases you will need a place to park near your hotels. So you should consult their websites. Usually they mention the nearest public parking.

When you visit a larger town or a city by car, you just follow signs for "Zentrum" (in Germany) or "Centrum" (in the Netherlands).
Most towns of a certain size (i.e. not that small that you can see that you are at the historic town center already) have parking routes with eletronic displays or stationary signs guiding you to the different garages available and showing available spaces.

If you are not determined to stay right in the historic center (esp. in big places like Amsterdam), you could stay at the edges of town (a bit like the motels/hotels in the US along the freeways), and take the train into the city.
You will notice these hotels when you, for example, look for a place to stay in A'dam and set the search radius to 20kms.
It's not a bad solution when traveling by car as you probably have come from an extensive trip and just need a place to crush first night. Next morning you are in central A'dam in 1/2 hour or less, and can ride back to hotel until early morning. The price for the hotel will be just a fraction of what you pay in A'dam proper, and you will also save premium parking costs. And you will find it much easier by car.

I would not do so in places like Brugge where you really want to stay in or in walking distance to the historic center.

P.S.
When I marked places with (visit) above this did not mean that you could always visit ALL of those on one day trip. It was meant more like options from which you had to chose.

P.P.S.
The route to central parking garage will obviously also in the Flemish region of Belgium be signposted as "Centrum" , "historisch centrum" or "binnenstad". And not just in the Netherlands.
The parking (garages) will sport the international sign of a white "P" on blue ground.

Here is a screenshot from Google streetview of a typical city intersection (in Delft/NL)
You will notice the sign that will guide you to a centrally located parking?

http://maps.google.com/maps?q=delft&...239.77,,0,7.28

I concur with one poster to spend more time in Brugge than Brussels. We stayed in Brugge for 4 nights in 2008. We had a car but opted to train to Brussels and take a bus tour of the city. This was plenty.

We also took one afternoon to drive from Brugge to the coast. They have some nice resort towns in that area.

Brugge itself is great. I recommend you watch the movie IN BRUGGE prior to going. It is with Colin Farrel. It takes place 100% in the city and is not a bad movie. You will get a good feel for the town.

Hamburg is another unique city located on the lake. We enjoyed our time there many years ago.

We always seem to enjoy the small towns in Europe more than the large places. You always forget the traffic jams when you are planning these trips....and they do have them! And they do waste your time.