You shouldn't miss Arches NP. There are a number of iconic arches to see including Delicate Arch. It's a 30-minute drive from Canyonlands. We toured Arches and Canyonlands in 1 day. I did Bryce in a couple of hours but I tend to race through things.
We were on a tour, so I’m not sure the best way to get there We’ve traveled quite a bit. Bryce is near the top of my list of natural wonders. Zion, which isn’t that far away, was nice, but not quite so striking as Bryce. We were there on a perfect day, so we hiked to the bottom of the valley, I think there are several paths. It’s like being on a strange planet (I guess we are a strange planet) we missed Arches. I hear iits great.
If you haven’t been to the Arizona parks or the Sedona area it’s worth it. Of course the Grand Canyon, but Monument Valley is interesting. Lots of commercials and movies are filmed there.
I would highly recommend hiking the Virgin River in Zion NP, a beautiful slot canyon. Distance is in and out, up to 16 miles. Both my wife and I thought it was great fun. Recommend early in the morning to beat crowds, solid hiking boots (very rocky), hiking poles (see very rocky), quick dry shorts/shirt/hat, and water/snack. The canyon narrows in several spots, meaning that the water deepens and you will get wet.
Stayed in St. George for about a week many years ago, nice place to stay. Close to Zion, a little further to Bryce Canyon. Bryce might be the most photogenic place I've ever been. Point a camera in any direction and you'll get a great shot. Bryce is relatively small compared to Zion. Sorry to hear your wife is hurting. Hiking is mostly what we did in the parks.
What is the timing of your trip? I think we went in early April (in fact we watched NCAA Tourney Championship games during the trip--that might've been the year UConn won men's and women's). Some of the days were warm, some were cold and stormy. Bryce in particular is fairly high elevation so winter lingers longer than some might expect.
One more note about St. George. Just outside of town is Snow Canyon State Park, rather spectacular but it is overshadowed because of the nearby National Parks.
I'm an avid hiker, so it's pretty much a given that's what I'm looking to do when I visit an area. However I think you can definitely enjoy Zion NP without doing a lot of walking around. Park at the visitor center, hop on the shuttle buts and ride all the way up and back down, staring up at all the beautiful multi hued cliffs and the river flowing beside the road.