One area of people's game which is typically weak is their in-court game and the footwork related to transition patterns (i.e., hitting approach shots, short balls). That can be worked on, regardless of how good your opponent is and it doesn't require big strokes. If you're better than your wife, you probably can enter within the first three feet of the back court and you probably can control your strokes so that you don't out-power her uselessly.
So, one option would be to practice entering the court and then retreating behind the baseline -- it's challenging physically and technically, but because you focus mostly on your feet and body position, you don't need to hit at challenging targets or try to hit very hard to benefit from it.
An other common problem with people is their tendency to play JIT tennis. They move to position themselves "Just In Time" to make a shot... ideally, you should be there as early as possible and instead of delaying your footwork, you should delay your shot. Yet an other problem that is related to this bad tendency is when you see people taking back their racket way too early. You can turn your shoulder and hold it on your side, but the tack back and the swing should be pretty late -- that's actually the part of hitting a ball that has to be accomplish Just In Time!
These two issues happen to make hitting slower balls A LOT HARDER than mid-paced rally balls. Your shot is only as good as your preparation allows... and hell do people prepare poorly when the ball is a gift!
So, movement and preparation are two things that you can easily work on without totally pulling your wife out of the court and out of her socks. I know that it's not as exiting as hitting a big serve or a good backhand winner, but it's important to be able to do these things properly ALL THE TIME. Work on getting a Federer-quality preparation for every ball (which is actually pretty demanding physically because you have to work A LOT to get an ideal body position on every shot, even against an average amateur) and you'll find it easier to play to your terms once you meet a better player.
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