You can get a painful sunburn even in subfreezing temperatures. Develop an emergency plan before you start your trip. Make sure everyone knows what to do if they become lost or a medical emergency arises. Take frequent rests or vary your pace to recover from strenuous activity spurts. Drink plenty of water.
Water is heavy to carry, but thirst on the trail is a hazard.
Don't run out of water. Never drink your total supply between refills. Backcountry water supplies are unpredictable. Treat or filter all water and rehydrate to your heart's content at the water supply. Bring a first-aid kit tailored to your outing.