Traveling around I have met many people from The Netherlands, so it was nice to finally see the country for myself. The rumors of everyone riding bicycles are true, as there are entire car lanes dedicated to them in some cities. Amsterdam has an amazing art scene and is quite the city to explore, which means you should definitely spend some time outside of the touristy areas (red light district). The canal tours can have some nice views, but opt for the smaller boats which can fit down the narrower canals and bridges.
Belgium is a beer drinker's delight as the Delirium bar in Brussels has over 3,000 kinds of beer in one location for you to enjoy. Also the fries (not "French fries!") are delicious and should be paired with local mayonnaise. Add to that chocolate, meringue, macaroons, and other local foods and it is a pretty easy place to put on some extra holiday weight. By all means if you are in Brussels go wait in line at Tonton Garby cheese sandwich shop for a delicious local experience! It's refreshing to meet someone so into their craft, and all sandwiches are tailored to your taste, each made by one man! Bruges is also a great little stop off and is easily navigated by foot. The Belfry tower is impressive and the town is great to spend a day or two wandering around.
Luxembourg was an interesting stop off because it is a less traveled country in the region. Most people opt to spend their time in the neighboring countries, but I found that my Iranian born hosts made my time there enjoyable. Much like Bruges, you will find Luxembourg city easily navigated on foot, but the rental bicycles are also a steal at one euro a day (provided you ride for under 30 minutes at a time, easy). The city itself still retains original architecture and walls, while the business district has a modern feel and some art museums which are worth a visit.
Northern France of course had some amazing wine and cheese. Stopping off in a small town (in my case Lille) lets you see France outside of the hectic sprawl of Paris. In Paris it is easy to fill your daily schedule with outdoor sights and indoor museums. The Louvre itself takes hours just to walk through, without any substantial stops to see what packs the walls and hallways throughout. Definitely give it a couple of days to enjoy! For those feeling more adventurous I would suggest finding a local to guide you through the catacombs. While there are on the rail tours where you will not get wet or potentially break laws, a headlamp journey through the underground will not be forgotten. Just be sure to pick a good guide who knows how to set up a proper picnic underground and don't forget the dry stuff sacks for your belongings!