Another option would be to take a train to Paris where the car rental offices (at least tthe Europcar one) apparently *are* open on Easter Monday and pick up a car there. There is a train leaving Calais at 11:32, arriving Paris at 14:02 and there are other possibilities both earlier and later.. The second class fare is ~47€.
It's about a 90 minute drive from Paris to Rouen. This option would have the added benefit of allowing you to stop at Les Andelys en route and visit the ruins of Richard the Lionheart's Château Gaillard on a bluff overlooking the Seine, definitely worth the visit. You could even spend the night at the lovely Hôtel de la Chaine d'Or close by.
One reason for booking early if you take the Rouen train is that the SNCF site is showing a special on the Calais-Rouen train for that day with first class fares the same as second class, 39€. I doubt that offer would be available for a last minute booking.
Good luck!
www.voyages-sncf.com