I have an extra day in Marseilles and would like to go to Carcassonne. Is that doable as a day trip? What transportation would I need and how would I arrange that? Thanks for the help!
The fastest direct trains take 3 hours each way, getting you to the destination by noon. Looking Up Train Schedules and Routes Online gives you the DB train schedule link and tips for using it (for info only, not to buy tickets for France).
Carcassonne is interesting, but not worth a 6 hour round trip to see, particularly since you'd only get to see it in the middle of the day when it's most crowded.
Look into Arles, Avignon, Nimes, or Aix-en-Provence - all much easier for a day trip. Of those four, my preference would be for Nimes or Arles, but of course experiences differ. I didn't get to Cassis, but that's also close.