Hi all -- I know the Navigo Découverte card has been covered thoroughly in these forums, but I wanted to point out a new "Forfait Navigo Jour" (Navigo day pass) that is available. (Well, "new" as of January 2018, but I did a quick search and only saw two mentions on the entire forum, and the most reliable information is currently also only available in French.)
The "Forfait Navigo Jour" allows you to load day passes on your Navigo Découverte card, for exactly the same price as the corresponding Mobilis tickets (€7.50 for 2 zones, €10.00 for 3 zones, €12.40 for 4 zones, €17.80 for 5 zones). So while it doesn't save you money vs. a Mobilis, if you already have a Navigo Découverte card and/or are always afraid of losing those little Mobilis or Paris Visite tickets like I am, you might prefer the Navigo Jour on your Découverte card. As with the Mobilis, this is a 1-day pass (i.e. 12:00 am to 11:59 pm) and not a 24-hour pass.
In addition, the RER to CDG is not excluded (on a zone 1-5 pass of course), which has been the case for the Mobilis in the past* -- the official information from the RATP (French only) -- states it's valid for all Métro, RER, tramway and train services, excluding Orlyval, SNCF trains requiring reservations, Le Bus Direct, VEA Disney, and the Open Tour and Cars Rouges tourist buses (terms and conditions, French only).
While that information page says this is ideal for "Parisians and residents of Ile-de-France who travel regularly," it explicitly states this is loadable on Navigo Découverte cards. The page also states that you can buy it up to 6 days ahead of time, although I'm not sure if this means you have to select the date of validity on purchasing or if it just activates automatically based on your first usage.
*Some information seems to indicate airports are now included in the Mobilis (see article 2, French only) since the introduction of the Navigo Jour, but there's a lot of conflicting info and the official RATP page still says that the RER to CDG is excluded from the Mobilis.