new itinerary to Normandy

My concern on the night of the 27th: where are you going to park the car for the night?

On the 30th, are you planning to stay in central Paris? You could return the car to La Defense or Orly and avoid driving in central Paris. You would then take the metro or taxi to your hotel.

Car rental prices are crazy right now, so that may help decide. Does the cost of keeping the car an extra day exceed the cost of train tickets from Caen?

It is certainly easier to pick up or drop off a car outside of Pairs. But the time it takes to actually pick up the car and get going is a time suck. If it were me, I would do whatever it takes to minimize the time inside of Paris, or in a rental car office. If you are going to pick up in Paris, and you have a parking spot, pick it up the night before so you can maximize your time in Normandy. If it were me, I would train to Caen on the morning of the 28th, pick up the car early, and drop it off late on the 30th before heading to Paris by train. That would give you one full day in Normandy (which is taken by your tour) and a half day on either end to see sights not covered by the tour, or sights you want to return to. It is not a lot of time in Normandy IMHO, but it should give you a day and a half.

As it may make a difference for using a GPS (or indeed, finding a place on a paper map!), please note the correct spellings of your destinations:

Giverny
Honfleur
Bayeux

Based on your time frame, I would do this: train from Paris to Bayeux on the 28th. You save a lot of time and money not picking up a car. There is plenty to see in Bayeux that day. Go on your tour on the 29th, which will be a busy day, so rest in Bayeux that evening. Train back to Paris on the 30th and spend your last night in Paris. If Giverny is a must, do a half day tour from Paris on one of your other days. That would save you the cost of the car. I don't think getting the car would be worth it for such a short and limited visit. You will miss Honfleur, but gain seeing Bayeux, which I think is a good trade.

Based on your time frame, I would do this: train from Paris to Bayeux
on the 28th. You save a lot of time and money not picking up a car.
There is plenty to see in Bayeux that day. Go on your tour on the
29th, which will be a busy day, so rest in Bayeux that evening. Train
back to Paris on the 30th and spend your last night in Paris. If
Giverny is a must, do a half day tour from Paris on one of your other
days. That would save you the cost of the car. I don't think getting
the car would be worth it for such a short and limited visit. You will
miss Honfleur, but gain seeing Bayeux, which I think is a good trade.

Excellent advice ^^
It would be a shame to go all the way to Bayeux and not have a bit of time to see the Tapestry and enjoy the town itself.

We spent just 2 nights n Bayeux, 1 night in Honfleur. While Honfleur was cute I wished we had given that night to Bayeux. We did have a car but it was start of a 2 week road trip for us.

Be sure to make reservations for dinner in Bayeux. IIRC many places were closed on Sun and on Monday we almost got shut out because we did not have reservations anywhere.

Personally, I consider a car a must in Normandy. There are just too many things to see, and practically zero public transportation. If you want to see anything not covered on your tour, or want to return to anything you wish you had more time at, it simply won't happen without a car.

Personally, I consider a car a must in Normandy.

I agree that car is a must for exploring the Normandy region but OP barely has 2 nights- 1 day already booked with a guided tour of the WW2 sites so the hassle and expense of getting a car for that short amount of time not really worth it. Doesn't really have time for exploring any more of Normandy. IMO.

Agree with Vandrabrud and Christine H.