We will be flying into Zurich in the morning of 9/15. We have 3 - 4 nights to spend before going to Geneva for 3 nights. Where is the best home base city to take day trips that is not too touristy?
The Berner Oberland is my choice, and I suspect most here would agree. Three or four nights is a prefect length of time, since it gives you a hedge against bad weather and gives you enough time to do a variety of walks, hikes, lifts, etc.
You can stay in Muerren, Wengen, Lauterbrunnen, or Gimmelwald. All four have their fans, and everyone who's been to the area has their favorite, but you can't go wrong with any of them. If hotels are too pricey, look into rental apartments. Each of the villages except Gimmelwald has a supermarket, so you can also save money on food.
Rick has an excellent chapter on the area in his Switzerland book. Except for its overemphasis on Gimmelwald (he has personal connections with people who live there, which makes it more interesting for him than for the rest of us), it has everything you need.
I agree with Harold, except you say "day trips". If you mean to other cities like, Lucerne, Bern, etc. I would suggest Lauterbrunnen because Wegen and Murren add to your day trips a bit. Even better, but touristy is Interlaken.
However, if you are interested just in day trips in the Alps, by all means pick Wegen or Murren as a base.