The first question is, when are you going? If it's in the spring/summer, you will have the choice of visiting the North Rim, which is an easy two hours from Kanab rather than the 5+ it will take to reach the South Rim. It's much quieter on the North Rim, and just as pretty.
Unless you want to hike into the canyon, at either North or South, your activities are more or less limited to driving around to scenic points and oohing and aahing. But there are few places on earth better suited for doing that. You could also attend a ranger talk; those are fun.
Both rims are at considerable altitude (the North is well over 8000 feet, and the South 7000), so you might want to consider how that might affect your wife. For that matter, most of Yellowstone is at 7000 feet or higher, so I would keep that in mind as well. Most people, even with lung problems, handle the altitude just fine after a couple of days, but I would ask your wife's doctor.