Spending 5 days in Northern France

What's your mode of transportation? If medieval is your thing, then you should see Guedelon, the castle which is being built using medieval tools and methods. It's taking 50 years; they are just over half-way now. Another is the city of Troyes, a regional capital of Champagne, which housed one of Europe's largest trade fairs twice a year throughout the medieval period. The fairs lasted three months each. The original core of the city, buildings and even medieval houses are intact. Much of Orleans was rebuilt after being destroyed in WW II. You should check a map to see if even the street plan reflects the original city. From my time there, I don't believe so. Most of the Loire is Renaissance and later. Some are built on medieval bases, but without the fortifications.

Three sights off the top of my head include Notre Dame de Reims, where the kings of France were crowned, and the Fontenay Abbey and Vezeley in Burgundy, if those fit your itinerary. I think Germany > Orleans > Chartres > Paris in one day is unrealistic.

I don't know if it worth your while, but there is a castle ruins in Coucy-le-Chateau near Laon. It once was the largest donjon (keep) tower in France. Unfortunately, the retreating Germans in WWI thought it would make a good artillery observation point, so they dynamited it. The French thought it was an act of barbaric vandalism, the Germans, military expediency. Anyway, the outer walls are still there and a pleasant little town is within it. If you ever read Barbara Tuchman's "A Distant Mirror", the site figures significantly as the home of the Sire of Coucy, an important figure in French 14th century history during the 100 years war.

I just learned that Aaron is traveling by train. So it has to be areas that can be reached by train, which means the larger towns. Troyes would still work as a day trip from Paris. As I wrote earlier, check street plans to see if any of the original plan is intact. Most of the larger cities have been re-done so the Medieval planning is difficult to discern.

Bets, I'm glad you mentioned that about Troyes. We decided the last time we stayed there a few years ago never to return because after an awful stay at a hotel that looked good on paper (ok, virtual paper - computer screen) we were in a bad mood and decided we hated the town. I haven't been right into the centre and will have a look the next time we pass by.

@Nigel, know what you mean. A horrible chain hotel almost ruined Montargis for us, but going back 2 years later showed us a lovely town with canals, bridges, etc. Hope you get to rediscover Troyes and its preserved center.

Since you have chosen to fly into Brussels (I trust you'll stop in Aachen on the way to Germany), it would be a shame to miss Bruges or Gent (pick one), especially since they are so easy to get to by train. I would remind you that after the cloth trade declined, Mechelen was the third-largest city in Europe. Do you have any interest in the UNESCO WHS beguinages in Belgium? I suspect that Strasbourg is not on your itinerary, but that's also easy by train. Perouges is also far West, but is perhaps the most unusual and picturesque medieval town I've seen - but it's not ideal for low-budget or non-car travellers. Do you have Dinan and St.-Malo on your possible list? Alas, everything in St.-Malo seems to have been converted to a restaurant. ... Here's another newsboard's post that might be of interest:
http://www.tripadvisor.com/ShowTopic-g187070-i12-k5607365-Medieval_village_to_visit-France.html