Re: Good trips for retired couple (health problem)
+1 for Gruyere - we took my (then) 92 year old grandmother there using public transportation from Fribourg two years ago, and she loved it.
The bus stop is very close to the touristy area, so the walk up wasn't a problem. She took her time, and rested quite a bit, and even managed to visit the entire castle. Her logic was that she probably wouldn't get to go back, so she should make the most of it on the day we were there!!
However, she is French, not American, and is used to walking everywhere she goes, so that might make a difference.