I use a camelbak Transalp for such entertainment.
Rain poncho, which can double as a shelter
Extra socks, 1 pair
A little more food than I expect to eat, maybe an extra MRE main meal
Multi tool
Water purification tablets
Waterproof matches, and a bit of dry kindling
First aid kit. Extra roll of first aid tape. Half-a roll of duct tape.
GPS, Map, compass.
Probably a few other things I can't think of.
Of course the last real day hike I took was above 8,000', and the nearest civilization was probably 50 miles. We were about 10 miles from the main road at any given time.
Usually when I go flyfishing, I'm out for the day, but never very far from anything. In that case, the only survival gear I carry is a pack of waterproof matches, a multitool, and a flask full of tequila.