I traveled with my mom when she was around that age, and in much the same way - I was the tour guide and she was along for the ride. I did all the work - ALL of it - and she paid for both of us, so I considered it an excellent deal. She did have some ideas where to go - "I haven't seen Italy, Austria, or Switzerland." But we also went to Amsterdam, Bruges, and Paris (she had been to Paris before but wanted to get up close and personal with the Eiffel Tower). I got to pick where to take her in her countries of interest. We were there almost a month.
How long do you have for your trip? Are your folks at all interested in the countryside, scenery, smaller towns or villages, or just in the urban centers? For example, might the Alps or Provence appeal?
For sure I'd include London, which is an easy place to start and has so much to offer. I'd personally pick somewhere in Italy too, although if your parents particularly like things sparkly clean, I'd choose somewhere in Switzerland or Germany as a third destination. If Italy, consider the vastly different characters of Venice, Florence, and Rome, and pick what will be the most appealing to them. I find the pace and clatter of Rome somewhat overwhelming, but there's no denying its wealth of top notch sights. Florence is smaller but more crammed with tourists these days - although if possible, go in shoulder season to avoid the worst of it - and maybe more of an art-and-history destination. Venice, as you know, is Venice, unlike anywhere else.
My mom was by no means fit, but she was also in good physical health and not limited in any way in her ability to walk. Basically I traveled at pretty much my preferred pace, and she was able to keep up. Since I was responsible for handling the luggage, that did slow me down a bit.
Consider modes of transportation and how taxing they may be. The train, if you take it during the day, is easy on everyone. Night trains can be hard going unless they are able to sleep on them - I can't really, so find them exhausting. Flying may seem easy, but there's all that getting to/from/through the airport business that makes it more of a hassle.
-artsnletters, also known as Road Crazy