An often overlooked benefit of the Paris Museum Pass is that you can pop in to a museum that you otherwwise would have skipped, as not worth the ticket price - or stay for only a short time, since there's no feeling of "get your money's worth." If a museum is crowded, you can leave and return another time. That happened to me once in June at the Louvre. In June the school take kids on lots of field trips, the Louvre was so noisy, I fled, came back around 6 p.m. on Friday and almost had the entire museum to myself. . . less than a dozen people in front of the Mona Lisa! Even if you break even, it's worth it.
Make a list of which sights on the Pass are open on which evenings, and which days they are closed, to get the most out of the pass.
The Navigo pass for the metro/bus system could be a good deal. The weekly pass runs from Monday through Sunday. If you can use it for at least 5 days, it's usually worth it. Sometimes it's nice just to hop on a bus for a stop or two, saving your weary feet for the sights themselves.