Hello Ben, We will be arriving in Paris on Sunday, September 13, and leaving Friday, September 18. Our intention is to purchase 2 Navigo Decouverte Passes for Monday thru Friday.
My question is in regard to tickets for the Metro on Sunday, our arrival day. If I understand correctly, individual ticket sales will be discontinued sometime this summer and a Navigo pass will be required. Since we will only use the Metro for 1 or 2 trips and a return that evening, what would be our best ticketing option for just a half day?
I'm going to assume you are not arriving by airport as that wasn't mentioned and you will be inside central Paris upon arrival.
Individual paper Paris Metro tickets will eventually be discontinued and an initial date of phase out was projected to be summer 2020, but there's been no hard set date published as of yet, just projections. In all likelihood, this won't come to pass in 2020.
If you're not making 4+ trips in a day, it's technically not worth buying an unlimited ride day pass but that doesn't factor in the utility/value of having a convenient unlimited ride single pass like Ticket Mobilis or Navigo Jour day pass, and not worrying about managing invididual use tickets/peace-of-mind of a unlimited use ticket. I personally value this freedom a lot and if I were to make 3+ Metro trips, I would use a Ticket Mobilis / Navigo Jour.
If paper Metro tickets are still in use, buying single rides as needed would be cheapest cost-wise. Otherwise I'd put a Navigo Jour 1-2 zone day pass on your Navigo Decouverte (which you'll have for your week pass, which I'm assuming you'd buy Sunday upon arrival, as there's no benefit of waiting till Monday).
In the case of paper tickets disappearing you'd be using Navigo Easy card then. The cost of 3 Metro tickets + Navigo Easy card costs (1.90x3 + 2€ = 7.70€) which is roughly the same as a Navigo Jour 1-2 zone (7.50€), so a Navigo Jour would be a good choice if you're making 3+ trips on Sunday/arrival evening.