Since you mentioned architecture: You could get to Strasbourg quickly by train (or Colmar in a bit more time) and rent a car to see some of the cute wine villages in Alsace. Colmar is touristy but gorgeous. Np car needed for Strasbourg, Colmar or Selestat.
There's a lot to see in Normandy, especially if you have a total of 14 days; you could take the train to Caen and pick up a car there if you want to see some of the D-Day sites (in which case I do recommend a van tour unless you are WWII experts) and don't intend to hit places east of Caen (Rouen, Honfleur, Deauville-Trouville, etc.)
I don't see anything wrong with your other options, either, though I would tend not to go to Lyon on this trip. I like it a lot, but I think your other possibilities might present a better contrast to Paris. That said, there is a lot to see and do in Lyon.
No one seems very excited about Brussels, though I think it has a lot of Art Nouveau architecture, which really appeals to me. Most folks push for Bruges, Ghent and Antwerp when Belgium comes up for discussion.
There are lots of places not all that far from Paris that would be enjoyable for folks who like to walk around attractive towns, as I do. Troyes is a town full of higgledy-piggledy half-timbered houses. It gets some French tourists, but few others find their way there. I saw not one tour bus. On the other side of Paris (could be combined with a loop through Normandy) is Le Mans, about which I've heard good things. Either of those cities is easily reached by train.
If you decide on Paris + Belgium, I'd try to get a multi-city ticket with Paris and Brussels as the gateway cities. It might not be more expensive than roundtrip to Paris, and it would avoid the need to burn a day of your trip looping back to Paris. The same might be said for Luxembourg, but I don't think it's served by many airlines, so a multi-city ticket might mean you'd have a stop on the way to (or from) Paris.