If this is your sixth trip to France, I'm guessing you've been to Paris at least once, probably several times, before. So you must know the sights and the city layout. If you'll have significant luggage (more than you'd want to haul around the city), I'd suggest getting yourself first to Gare d'Austerlitz to check it, see here. Your next move depends on weather. Assuming that it's nice, here are some ideas:
- Maritime option: head north across the street to the Jardin des
Plantes and have a look around, then board the Batobus for a
ride down the river to your favorite stop. You could get an all-day
pass and hop on and off to different places if you want. But the
service closes at 19:00 in the "off peak" season, which is probably
Terrestrial option: head north on foot across the Jardin and along
the river to the Ile St-Louis and Ile de la Cite. Stroll, café-stop,
people-watch. Cross the bridge to either the Latin Quarter or
Rail option: take the RER C or Metro lines 5 or 10 wherever you want
and do whatever you want. This is probably your default option if
the weather's bad.
Something-completely-different option: Cross the Austerlitz Bridge
and continue on Ave. Ledru-Rollin to Ave. Daumesnil, where you'll
find an entrance to the Promenade Plantee, a former rail line above
the streets that goes from Place de la Bastille out to Vincennes.
Walk a few blocks to Bastille, or as many as you like out the other
way. Return by foot or Metro, depending on how far you've gone and
whether it's started raining.
Those are just a few ideas. I'm sure you must have others.