We have a booking for Varenna but my 75-year-old friend thinks it might be too hilly for her. I believe that Lecco or Como City might be a better choice. We will be staying for 3 nights and then head off to Milan for 3 nights.
Lecco takes the cake. Menaggio also works and is more centrally located on Lake Como.
Menaggio has a lake promenade that is completely flat. But be careful with hotels as some can be up a hill. But overall much more area is flat than in Varenna. Like Varenna, Menaggio is on the midlake ferry lane which is a real advantage trying to get around the lake.
I've stayed in both Lecco and Como City and enjoyed them both, and I've also stayed in Varenna. I think the Menaggio suggestion is a great one.