My granpas used to spend a week in Montserrat every year, until my gradma died, well passed their 80's....

Of course some walking is involved, but if you stick to the area closer to the Monastery, it should be OK fro everyone. You can take as well the funicular to Sant Joan peak and enjoy the views from here. As far i remember, some steps involved, but nothing extreme

Go to the FGC train station in Plaça d'Espanya (it's an underground station), and ask there for the combo tickets (train+funicular or cable car). I like more the views from the cable car, but some find it scary. There is usually English speaking staff that will help you, there are always tourist buying these tickets there

There are some tickets including the museum (great if you're art lovers), or lunch... that may be worth it