First aid kit. Map and compass. Trowel for sanitation and fire. Some people also consider mosquito repellent and toilet tissue essential. Be on the lookout for them.
Examples are fallen trees, steep drop-offs, falling limbs and trees, fences, and swift streams.
Poison Oak, watch for rattlesnakes. They appear anytime, anywhere. Normally they only want to get away from you, but be alert, particularly around water sources and shaded areas. Only water from developed systems at recreation sites is maintained safe to drink. Open water sources are easily contaminated by human or animal waste.