I would recommend shooting RAW to take full advantage of what that great sensor can do. If you don't want to buy Lightroom or Aperture, then it is probably best to shoot jpegs. Try out the Vivid setting. One option is to shoot RAW+JPEG, so if you decide to get into the whole RAW processing thing later, you'll have all the RAW files. Photographers think of RAW files as being the "digital negative", i.e. something NEVER to throw out. Check out http://www.kenrockwell.com/canon/s95.htm for a ton of info on shooting with this camera (although I disagree with his view of shooting RAW).
Here is my two cents: Shooting in P (Program) mode is what I did, and then I used the control wheel on the back to adjust exposure. Set it up so you get the live histogram view, and make sure you are not clipping the highlights or crushing the blacks. Rotate the wheel so the data not crushed against either end of the graph, and then snap! Set up the camera so you get the view after you take the picture where the clipped portions of the picture flash (sometimes called blinkies), then you'll know if you over or underexposed. Sometimes scenes are too contrasty (like pointing the camera at a setting sun, in order not to blow out the sky, the mountains below will be silhouetted and near black), so you'll have to accept dark shadows. In general it is better to underexpose shadows in order not to get overexposed highlights. The control ring around the lens is best set to control ISO. Always shoot the lowest ISO you can, without getting the camera shake warning (usually shutter speed below 1/30). If you get a camera shake warning, bump up the ISO until it goes away. In P mode the camera will pick a faster shutter speed as you increase the ISO. You can also just put the ISO on Auto. This is all basically digital photography 101 and applies to any camera. Check out some articles about understanding exposure on photo.net.
I used an S90 (almost identical to the S95) on the JMT last year, and I just put it in my back pocket, and it was fine. Cameras in general are more durable that we care to realize. It got a few scuffs here and there, but who cares? One thing is these cameras have short battery life. Expect no more than 250 shots per charge, and don't review your images in camp every night. Not sure if you'll be able to upload photos during your trip, but 8Gb cards are about $8 now and weigh nothing. Load up on them!
Of course none of this addresses WHAT to point the camera at. Have a great trip!