The one time that I got stuck I was free climbing (without a trail) up some big boulders in Red Rock. Once we had got to the top we realized that we couldn't go back down the way we came up so we continued to go up further and eventually got to a point where we could continue but we would have to scale a boulder, on it's side, over a 150 foot drop. If I had been prepared with the bag I now carry we would have been able to use rope as a safety harness. This is the content of my bag, which is the bare minimum I would suggest just because the day I got stuck it was supposed to be an out and back hike but apparently was not:
-My bag is a camelback, and a backpack, you have the option to have just a camelback however if you are going to carry something on your back anyways, it's easiest to just have everything in one place.

-Rope, not too thick and not too thin, I carry 1/2" rope with a 300 -400 pound weight resistance that is about 20-25 feet long
-Wool gloves and socks, these may not be the most comfortable but they keep your hands and feet very warm and if they happen to get wet, the wool will actually retain the water, allowing your body to warm up that water and will continue to keep you warm
-Waterproof bags to keep your cell phone, cash, i.d. and any other equipment that you do not want to get wet, dry - also, if you are going anywhere that water will be in abundance, a waterproof bag large enough to hold your clothes. (that way you can place your clothes in the bag, walk across the water, then put them back on and don't have to continue hiking in wet clothes)
-extra water, if the hydration pack is full, between 1-2 bottles
-snacks
These are the minimum of what should be taken. Each hike is different so it can be adjusted accordingly and items can also be added.
Hope you have fun getting ready!