The list of the top cities to visit in France that aren’t Paris begins with Lille and goes all the way south to Nice: Mon dieu, there’s a lot of France that isn’t Paris! Paris can be cold (both climatologically and interpersonally), it’s expensive, the weather’s terrible — yes, it’s beautiful, but so is Lyon! And Lyon’s cheaper, friendlier, and both sunnier and snowier!
The news is basically all good in terms of the best day trips from Paris by train: There’s a wealth of attractions within Ile-de-France, the administrative region that includes Paris and its suburbs — nearly all of these are accessible via the RER or Transilien commuter trains. (And it’s even better if you get a Navigo pass for the week or month — with an all-zone pass, which would include transport to and from Charles de Gaulle airport, you can travel for free on all of these lines.)
So what are the best flea markets in France? The French take their flea markets very, very seriously, and these events are hugely popular. Plan in advance for accommodations or risk staying out of town.