Welcome to this newsboard cwallace, with your first post.
Month of year?
France is a huge country, and it's unclear what you don't "need" to see. Belgium is a big enough country that you can't see wide parts of it in just a week. It's a unclear what you mean by "bridges." Belgium does have good system of unreserved local train service that covers many places, including some smaller cities and towns. Among the main interests people have in Belgium are medieval town centers, alas, surrounded by prosperous postwar sprawl.
In most cases, both France and Belgium are often visited by taking multiple daytrips by train from a base city. Here are some examples, just to give you an idea of what many posters on this newsboard do.
https://community.ricksteves.com/travel-forum/belgium/day-trips-from-brussels-768c7c85-75b9-4374-ad13-ae36967bc521
https://community.ricksteves.com/travel-forum/belgium/day-trips-from-brussels-besides-bruges-and-ghent
https://community.ricksteves.com/travel-forum/france/loire-base
https://community.ricksteves.com/travel-forum/france/day-trips-from-paris-bfdcdec9-1a74-4e3e-8f0c-453f3da8da54
Your perfectly good idea is heavily impacted by the radial nature of most rail networks. This is particularly true of France, where Paris (for instance) has many train stations, each one serving "that direction" of the country from the station. Even locally, to visit more than one town in a day can require choosing from towns that happen to be on the same rail line. For example, despite its popularity, Bruges, Belgium is a "spur" journey from Brussels, and even if it weren't a rich, full-day destination, would take time to get there and back to Brussels (or Antwerp, a place I much prefer to Brussels.)
https://www.ricksteves.com/travel-tips/transportation/trains
You mention pubs, but not "beer". I say that because abbey brewing (requiring a car) is a notable interest of many visitors to Belgium.