A mountain bike that doesn't have suspension and is fairly lightweight. Or better yet IMO, a cruiser with tires smaller than 700C. My wife won't even look at a bike with 700C wheels anymore.
https://www.trekbikes.com/us/en_US/b...bikes/c/EB600/
These probably look awful to those used to the aesthetically pleasing and racy looking road bikes, but for leisurely rides they are very comfortable.