I'll be renting a car in Caen or Bayeaux to travel through Normandy and the Loire Valley. Is it fairly easy to drive from Amboise to Paris CDG on a Friday afternoon/night to return the car, or am I better off returning the car outside of Paris and taking the train to CDG and the surrounding hotels?
It's definitely easier to take the train from, say, St-Pierre-le-Corps near Tours, where you should be able to return the car. This website will give you train schedules, and this one will help with driving routes and times (which it will underestimate because it doesn't consider traffic or stops). To avoid driving, you'll need a train to CDG that fits your schedule, and you'll need to buy a ticket as well as pay that day's car rental charge (but not gas or tolls). I didn't do a time comparison but it's easy enough with the links above.