In response to acraven's post, I'll mention there is a walking tour of Canterbury on the tour - ours happened on the way to and from dinner, yours may be timed slightly differently.
The first morning will include a tour of the cathedral and a tour of the stained glass workshop with your group split in half as the workshop is tiny. The workshop was one of my favorite events on the tour...absolutely fabulous. I went back in to the cathedral afterward to see the stained glass panel our workshop guide had done. It's one of the new ones installed in the Cloister and it was so special to see it after spending time with her. As an aside, when my brother and I were in the cloister area a Harris' Hawk flew thru followed by his falconer. They were on pigeon patrol. (Hawk was named Prince Harry, his falconer was Conan!!)
I really enjoyed the Saint Augustine's Abbey (did not plan enough time here!) and wish we had had time to walk a bit further and see St Martin's Church as well. It is the oldest parish church in England still in use.
We also enjoyed a couple of walks along the River Stour to the Hambrook Marsh. We walked along it one afternoon, then my brother and I got up early to go out there birding. I was surprised at how many locals stopped us to talk. They were walking their dogs and seemed to want to give us advice on what to see in the area. It was a lot of fun!
I would plan on having a collapsible walking stick with a rubber tip (not the carbide tip) for the walk up St. Michael's Mount. The path is very cobbled and the rocks are kind of set upright so the pointy side is up. If it's raining it will be very slick. The tide was in when we got there so we took the small boats across, then walked back on the causeway as the tide had receded.
This is a fun trip and I know you'll have a good time.