Just 1.5 days in Paris--What to do?

Paris is such a big walking city that the answer to this really depends on your mother's mobility.

She's still very mobile. She can walk quite a bit, and that was how I envisioned spending most of our time. I'm wondering about which walkable neighborhoods might be best for introducing her to Paris.

FYI, there is an elevator at the Arc de Triomphe (stairs up the last about 45 steps) for seniors, disabled people and parents with strollers -- just ask. Separate entrance after you get tickets.

Five metro rides here, so share a carnet of 10 tickets between you over the 2 days.

Sunday afternoon
- Sacre Coeur near your hotel (steep climb up stairs or take the funicular)
Metro Line 2 from Anvers to Etoile
- Arc de Triomphe
Metro to return to your hotel after dinner

Monday
Metro Line 13 from Guy Moquet to Invalides
- Right Bank Walk: Invalides >> Pont Alexandre III >> Grand+Petit Palais >> Place de la Concorde >> loop thru Tuileries Gardens, possibly visiting the Orangerie >> Madeleine >> Opera
Metro Line 7 from Opera to Pont Neuf
- Seine River Cruise - 1 hour with Vedettes du Pont Neuf http://www.vedettesdupontneuf.com/
(Get a great view of the Eiffel Tower whilst on this cruise)
- Notre Dame
- Left Bank - Latin Quarter & St Germain
Metro to return to your hotel after dinner

Les Vedettes du Pont Neuf are a better choice for a Seine cruise. Best time is right around sunset. An evening concert a Sainte-Chapelle. Paris after dark - but remember that in summer it gets dark really late. I've heard that Rick has a taxi tour to see the best illuminated sights. I can't imagine going to Paris without a quick stop at Notre Dame. It is close to Sainte-Chapelle and Les Vedettes du Pont Neuf so that might be a good grouping.

On Sat-Sun, some of the streets in the Marais are closed to vehicles. Lots of Parisians come out then. There are some ancient buildings, the lovely Place du Vosges, lots of cafes and restaurants and trendy boutiques.

For your Seine cruise try to take one as the sun is setting so you can view the route in daylight and at night with the city bathed in lights. We've taken both Bateaux Mouches and Les Vedettes du Pont Neuf cruises and our opinion is the latter is a much better choice. Perhaps the most important aspect about such a short visit is to find out if there is anything she has always secretly wanted to see in Paris and put that at the top of the list.

These responses are all immensely helpful. Thank you so much!

My suggestions:
first 1/2 day, Sacre Coeur and Montmartre area
full day: Walk Ile St. Louis and Ile de la Cite - go into Notre Dame, (St. Chappelle would be fabulous but lines may be long)
Stroll banks of Seine, stopping at interesting shops or cafes.
See the pyramid outside the Louvre, walk down Tuilleries to place de Concorde
Stroll Palais Royale to the Gallerie Vivienne, charming covered gallery, good restaurants, shops
Cafe Nemours on Place Colette outside of the Palais Royale, across from Louvre is wonderful place to sit and people watch
Take sunset or nightime cruise of the Seine -- be sure to see the Eiffel Tower light show when it's dark.
Going to top of Arch de Triomphe (via the elevator) would be nice if you can work it in.
OR, you could take the hop on hop off bus and easily see a lot of the main sghts.
Note, taking the metro everywhere may not be that wonderful, there can be a lot of walking underground in the stations to get to
the correct train, and no air-conditioning. Lot of time and energy can be expended there -- for only a 1 1/2 day visit -- I'd stick to buses or cabs.