Ten things to always have handy when exploring the outdoors.  The “Ten Essentials” first became a common idea in the 1930s, and has evolved ever since.  Many organizations and agencies promote their own “Ten Essentials” variations.  Here is a reasonable version:

Navigation

Minimally, this means a topographic map and compass.  A GPS is a convenient supplement (but not a replacement for) a map and compass.

Extra water

The water you’ll need, plus some.  Or a method to gather and treat water, if fresh water is available in the area.

Extra food

The food you need, plus some.  An extra granola bar or ramen noodle pack can get a lost hiker through an unexpected extra overnight.

Rain Gear

Although it can happen at any time of the year, being caught in a mountain rainstorm during monsoon season can be life-threatening.  Temperatures drop by 40 degrees within minutes, hail can stack up inches high on the ground, and soaking rains can last for days.

Sun protection

Sunscreen.  100% UV glasses.  And perhaps a wide-brimmed hat, depending on the climate and location.

Knife

More hikers carry a Swiss Army Knife than any other single tool.  But a fixed-blade knife is generally more durable, more hygienic, and safer.

Fire starter

Cook food.  Get warm.  Treat water.  Start a signal fire.  Waterproof matches and/or a durable lighter are the most reliable.

First Aid kit

Pain reliever.  Treat and prevent blisters.  Pepto Bismol.  Tweezers for splinters and cactus needles…  A small kit can be the difference between happily bouncing down the trail, and miserable trudging.

Flashlight

If something goes wrong, it could likely result in after-hours hiking or an unplanned night out.  While it might be best to sit and wait for morning and while night-hiking is often better without a light, it’s sometimes necessary to read maps, take notes, find the proper first aid kit tablets, or prepare dinner.

Signal Device

A whistle is probably the most effective and convenient, make yourself known to search and rescue teams that might be on the ground.  In some situations a mirror might be better.  Flares are essential for boats on open water.  Don’t depend on cell phones or other electronics: the batteries go dead, they are easily broken, and cell reception is spotty.

And the 11th Ten Essential,

Insect protection

Discomfort.  Diseases.  Sleepless nights.  All prevented by a little deet or other effective bug repellent.

Adapt each of these to the climate, elevation, time of year, and type of terrain.  Navigation tools on the water are generally not the same as land-navigation tools, for example.  “Extra clothes” should be treated totally different in the winter than in the summer.