We will be travelling from Gare Du Nord to Disneyland on 14 August, noting the closure in place, one way to Disneyland. On Sunday we will travel into central Paris and also spend Monday in Paris so may make some metro journeys that day. What would be our best ticket option? The Navigo Decoverte, noting there is a 5 euro fee for the card or is it cheaper to just pay as we go using a one way billet ile de France for the journey to Disneyland and back into Paris on the Sunday and then just using carnet tickets on the Monday as we dont know yet how many metro journeys we would take that day?
Since you're not sure how much you'll ride the Metro in Paris, sharing a carnet of single Metro tickets is a good option for Monday. Using RER/Billet Ile-de-France tickets for Disney-Paris is also the cheapest option if you won't make any other trips outside of zone 1 / Central Paris. As long as you start your Paris-Disneyland trips before 9pm each night, you shouldn't run into any RER A line maintenance closures and can take the RER A train directly between Disney and central Paris.