Eat whatever you want, you'll be hungry.
I found navigation to be time consuming in the Netherlands there are bike paths everywhere, well-signed but unless you are very familiar with the area, you have to keep checking your map to see which path you should be taking. Get good maps.
I didn't worry about panniers in the smaller towns, only in the big cities. I don't think there is a lot of point in locking panniers to the bike, somebody could just go through them and take what they want anyway.
Kinderdijk is worth seeing, I liked the old Hanseatic league cities like Kampen and Deventer.
I've heard Brussels is a nightmare to cycle through, you might want to consider a train into the city.
Sounds like a good little trip, have fun.