Zion will be tougher hiking. It is a gorgeous drive in from the tunnel on Hwy 9 down but that is the best part of it....you will have to take a shuttle bus once down in and all you get is a stiff neck looking up; which is one of the reasons it is not high on our list. You will not have an easy hike to get views - particularly with your knee and wife's (not unusual) situation.
Which is one of the reasons we like Bryce so much better. You can drive to lookouts. Hikes there are down and you come back up. Some easier than Zion but still a hike. Beautiful views from the lookouts. And there is not the "edges and ledges" factor for your wife.
I would skip the North Rim of the GC and go to the South Rim. There is no walking much there to oooh and aaah. And you only see about 10% of the Grand Canyon and all viewing is right into the sun. And quite a boring drive to get to. If you really want to SEE the Grand Canyon you have to go to the South Rim. There you will truly see the Grand Canyon and you will have nice walking along the Rim Trail and lots of places to drive to for lookouts as well as a shuttle out to the various spots to the west end.....and the shuttle stops you have places you can take shorter walks to see the vastness.
We will be doing those places once again this Fall - Moab, Bryce and South Rim - been there many times. Have been to Zion and North Rim only twice; second time for each as we wanted to give them a second chance. But not again.
Enjoy. Be safe.