Paris in March for 5 days.

After five trips, you've probably been to Versailles, but if not that would be a good candidate (RER C). Monet's house and garden at Giverny (Gare St-Lazare to Vernon, then shuttle bus) would be another (though March wouldn't be the optimal time for the garden). Chartres is an easy day trip (Gare Montparnasse). Fontainebleau, Vaux-le-Vicomte, and Malmaison are nearby chateaus. Reims (TGV from Gare de l'Est) is doable though might be better as an overnight. St-Denis Basilica is beautiful and historic (Metro line 13). You'll get more ideas from others.

Ina Caro's Paris to the Past describes 24 historic sites reachable by train from Paris. She considers them day trips though a few might be better as overnights. This would be worth perusing.

If you know enough French to function as a tourist, after five previous trips, I'd recommend traveling on your own, though more may be involved than just a train ride (as for Giverny or Vaux-le-Vicomte). A good tour can give you a lot of information and save you dealing with logistics, but you lose control of your schedule and pay much more than on your own. Your tour companions can be enjoyable, or not, or a combination. So what's "better" depends on your preferences, budget, and language skills.

Reims is possible as a day trip (cathedral, champagne cave tours/tastings) - even better as an overnight.

I believe Monets gardens in Giverny are not open in March. Check before going.
Chartres is a lovely day trip from Paris with a beautiful cathedral and charming town.
Lisa from Tampa

I did a day trip to Rouen solo and enjoyed it very much, however it was in summer when it was light out till 10 so I had lots of time to wander town before heading back to Paris. It was only a bit over an hour by train as I recall, maybe a bit more.