Belgium and Lille, or Lyon and Annecy?

Since you say you are going first to Nice and then to Paris you should take the train north to Belgium; if you have a week split it 3 in Bruges and 3 in Ghent or Antwerp (throw in a day trip to Brussels to see the Grand Place but you should do some research on what each town has to offer). Those towns are all well connected by short train rides.

Lyon is right on the way between Nice and Paris, if more of France is desirable. I've booked the Away Hostel in Lyon, which certainly would be foodie choice among the cities you mentioned. Otherwise train transportation from Paris to Belgium and even The Netherlands is easy. Even the hostels in Netherlands are frightfully expensive this summer, less so in Belgium.

Lyon is a nice city but it can't compare with Bruges or Ghent. So if you have time I'd visit Bruges.

Lyon is very under rated in my view. Loads to do, great food scene and some stunning architecture....and Annecy is beautiful so I would reocmmend this route. Belgium is nice to live boring to visit. Bruges is beautiful but touristy so Gent is a bit more authentic, and I also like Lille which is a fun city with some cool bars..at end difference is whether you like cities.

Belgium and Lille would be my choice. And I absolutely loved Annecy, one of my favorite places in France (or even in Europe), but I was there 10 years ago and it was crowded but tolerable. I've heard horror stories about what it's like now, especially in Summer.

I stayed in Brussels and visited Antwerp, Bruges, and Ghent. My favorites were Ghent, Bruges, Antwerp and if I went again I would stay in Ghent and visit the others. They are all pretty close and short train rides apart so easy to visit all four in a week. I've not been to Lille but have heard it's very nice for a short visit.

Europe in summer can be miserably hot----so I'd suggest le grand nord and doing Belgium and Lille. But really do the surrounding area of Lille, too.

Given your interest in architecture I would suggest Belgium and Lille. I have been to Lille twice , both times as day trips in the summer.

If you want to see more of the architecture in Northern France , I would recommend starting in Arras, above all, the centre ville. My trip this summer will include a day trip to Arras which I've not seen in almost 2 decades.

I don't know of the hostel in Lille but there is one in Arras beyond the centre ville. From the Arras train station, ca. 50 min walk, all level ground.

I am not familiar with any hostels in Lille, surely there are some. But there are some good deals on hotels in the city. Try an Ibis styles or there's the Kanai in the centre ville. Hotel Lille Europe usually has good deals, too, near the Lille-Europe gare and attached to the mall.