A big advantage of the valley floor walk over any others is that there is bus service (twice an hour for most of the day) all along it. So, if she gets tired, she can take a bus back to Lauterbrunnen.
Along the valley floor, about halfway between Lauterbrunnen and the lift station to Gimmelwald/Mürren/Birg/Schilthorn, there are the Trümmelbach Falls, which are largely inside rock. There is some climbing involved, but I believe parts can be seen without climbing (an elevator takes you up to the start, and you can walk down and still see much of it, without having to walk up).
If you do get good weather, do splurge for the Schilthornbahn and/or the Jungfraujoch. These are specifically designed not to require climbing (in effect, you're paying all that money for motors to do all the work; you just enjoy the views). Even with a Berner Oberland Pass, they're not fully covered, and they're not worth it if the weather is not good. Alas, that was my situation last week, and in October the weather is even iffier.
If you're not sure about the weather or just want to save money, the view from the top of the Allmendhubel (reached by train from Mürren's upper road) is great; you get to see the three peaks of the Eiger, the Mönch, and the Jungfrau in a row. I see it's fully covered by the Berner Oberland Pass, and is cheap at full price in any case, so, unlike the Schilthornbahn or the Jungfraujoch, you haven't lost much time and money if the weather turns bad once you get up there.
The Ballenberg Open Air Museum was quite interesting, but will be much less so if your mother can't climb stairs. These are old houses, and you can go into almost ever part of them, but they haven't in any way been retrofitted to be made more accessible. If she can climb a flight of stairs at a time, she will be fine here. While this is promoted as a "rainy day activity," be aware that the paths between the houses get very muddy in the rain (ask me how I know). So, while it's great when it's cloudy or lightly raining, it's not so much fun in the pouring rain. With a Berner Oberland Pass, the transit there is covered, and you get a 20% reduction on admission (very high otherwise).
She might like a boat ride on Lake Thun or Lake Brienz. Again, these boats, and the trains from Lauterbrunnen to them, are included in the Berner Oberland Pass, so you can decide on the spur of the moment. If it's really raining, however, you won't see anything (again, this was my situation; from the train station I could see that visibility was zero, so I didn't bother taking one).
All of these, and much more, are covered in Rick's Switzerland book; I hope you have this.
