For what it's worth, I would not do day-trips there; way too much time and money wasted traveling back and forth. Instead, I'd spend two or three nights in Bayeux and use it as a base.
I think that this is great advice. If you want to go to Normandy, spend a few days and really see it. You´ll have the time.
Get a Navigo Decouverte transit pass for 22.80 EUR each, plus a 5 EUR initiation fee. Also bring a 1" by 1 1/4" passport type photo that will be attached to the pass. Can be a selfie printed off on a printer with the reduction set to the right size, or a photo copy of your passport photo reduced to the right size. It goes from midnight Saturday night to midnight the following Saturday. It includes all RER, Metro, Bus transport in the Paris region, including the airports, Versailles, Disneyland.
Is the Navigo Découverte really a pass or is it not just a reusable card costing 5€. The card must be loaded with a fare to be usable. Fare options are 1 day, 1 week (starting on Monday, ending the following Sunday), 1 month, or 1 year. Given your planned itinerary, it would be a mistake not to take advantage of Navigo fares.
For the days not covered by the Navigo pass, just buy a carnet for metro/bus travel in the city.
While purchasing a group of 10 tickets will save you money over the ticket´s individual price, for métro/bus/tramway/funicular travel in Paris, you want the ticket t+, available at 1.90€, 2.00€ from a bus driver, or 14.90€ in groups of 10. A carnet is not really a ticket is it? A carnet is but a group of any 10 tickets which may or may not be tickets t+.
I don´t think that when using the English language menus at ticket kiosks, the word carnet ever appears. It might be better to look for the ticket t+ specifically if that is what you want.
If you run out of Navigo Decouverte money, you can buy individual tickets from the Metro machine. If you take the Bus, however, you may not use them for transfers.
I believe it is just the opposite. Only tickets t+ purchased for 1.90€ individually or in booklet form of 10 for 14.90€ may be used for bus transfers. Ticket t+ purchased from the driver at 2€ may not be used for transfers.