An itinerary for a seven days long, very tiring and hectic but exhilarating backpacking trip starting from Marseille in France to Brussels, Ghent, Bruges, Antwerp and Liege, all in Belgium; Rotterdam, and Amsterdam in Netherlands; and Luxembourg City in Luxembourg and back to Marseille.
If you have those two more days in your itinerary, I would highly recommend spending more time in Netherlands and a visit to Haarlem for sunset.
The trip cost mentioned excludes the cost of Eurail pass which I used through my travel. I bought one for €1120 which gave me free access to most trains and highly subsidized rates (€3 – €15) for high speed trains. I used this pass to travel for about 45 days visiting 24 countries taking over a hundred train journeys.