Navigation: Map and compass/GPS. Know where you are going and how to get there (and back).
Sun protection: Sunscreen, sunglasses/ski goggles, lip balm. The sun is much stronger at higher elevation and it is possible to get burned any time of year.
Insulation: Waterproof/wind gear and extra clothing. Layers are helpful – stick to tech fabrics that wick and avoid wearing cotton.
Illumination: Flashlight/headlamp. A light source is helpful for finding your way in the dark and signaling for help.
First-aid supplies: A prepackaged first-aid kit combined with the proper knowledge will be useful in case of an injury.
Fire: Lighter/matches/fire starter. A fire can provide warmth and be used to signal for help. (Caveat: In drought years there may be a fire ban or restrictions in place due to an increased risk of wildfire. In this instance, a fire is not recommended).
Gear: Knife or multi-purpose tool and a repair kit. You never know what you may need to fix in the backcountry, and tools such as a knife (and duct tape) can help in a given situation.
Nutrition: Extra food. If you adventure takes longer than expected, you want to be sure that you do not get hungry.
Hydration: Extra water and a way to purify it. Staying hydrated is important, and even more so at higher elevations.
Emergency shelter: Tarp, bivy sack or emergency blanket. It may be necessary to have a form of emergency shelter, whether it is a late afternoon summer storm that rolls in or a challenging hike that is taking longer than expected.