Paris Metro Pass

The Pass Navigo is a weekly pass, Monday-Sunday, 20 euros/week in zones 1-2, plus 5 euro set-up fee, and a photo. The one-day Mobilis pass costs 6.60. Or, a "carnet" gives you 10 single tickets for the price of 8 (13.30) to use whenever you like, or split between a group.

The Pass Navigo is restricted to residents but the Navigo Decouverte
can be bought by anyone; be sure to use the right nomenclature as
sometimes a ticket agent will pretend you are asking for the Navigo
and tell you you can't buy it. (happened to us once) It is the best bet
if you are there during 5 days between Mon-Sun when it runs.

If your trip laps over a weekend then it won't work for you and
probably the best bet is to just get a carnet of 10 ordinary tickets
for 13.70 and get another when you run out. These tickets can
be shared among the group. The mobilis is a good idea if you
plan a side trip to Disney as the cost is about the same as two
tickets to and from Marne le Valee. If you have a day when you
will be using the metro or buses half a dozen times, then the
Mobilis day pass will be a good idea.

Since Paris is really a great walking city, you can organize
your travel so you don't need to ride underground often --
just to get to more distant points like the Eiffel Tower or
Montmartre. Thus the carnet is usually the best way to go.
The Paris Visite is a tourist product that is almost never a
good idea; very overpriced for what it delivers.