Tell at least one person your planned trailhead, route, destination, and when to expect you back. Allow some buffer in case things take longer than expected. If you’re late, they should call Search and Rescue. The county sheriff, often assisted by volunteers, is responsible for search and rescue, though calling 911 works.
Consider buying a CORSAR (Colorado Outdoor Recreation Search and Rescue) card. The fee for the card goes into a fund that reimburses Search and Rescue teams for their expenses. Hunting and fishing licenses pay into the same fund. CORSAR info
food (more than you expect to need)
water (and a way to purify more)
sun protection (sunscreen, sunglasses, hat)
extra clothes (layers, gloves, warm hat)
map and compass (and know how to use them, even if you have a GPS)
light (flashlight or headlamp)
repair kit (duct tape, knife, tools)
some way to start a fire
emergency shelter (space blanket, bivy sack, tarp, or large trash bag)
some way to signal for help (whistle, mirror, light)
And, as Gerry Roach says, don’t get separated from your lunch (or other gear).
Don’t leave someone behind, especially if they are having trouble. Don’t send someone back alone, unless necessary for going for help in an emergency.
Stay on trail and on the designated route. The standard route to a destination is the best route. Getting lost is easier off-trail, and lost people off-trail are harder to find. Going off trail, especially in sensitive areas, damages the environment.
Check the weather forecast before you leave. weather.gov, can provide point forecasts. Get the forecast for a nearby city, then click on the map at the location you plan to be at. Be sure to check the elevation that the forecast is for, also. Then watch the weather while you are out. Storms can develop quickly.
To paraphrase Ed Viesturs: reaching the summit (destination) is optional, returning to the trailhead is not.
Travel and camp on durable surfaces
Dispose of waste properly
Leave what you find
Minimize campfire impacts
Be considerate of others
Going off trail damages plants, leads to erosion, and hurts nature and the resources that we are there to enjoy.
Don’t cut switchbacks.
Go through muddy spots on trails, not around them, to avoid widening the trail. Avoid using trails when they are muddy to avoid creating ruts.
Go single-file unless on a wide trail.
When yielding or going around other trail users, stay on trail if at all possible. If not, step on rocks or other surfaces that won’t be harmed.
Pack out all trash, including food waste or other biodegradable items, which take a long time to decay in the dry Colorado climate.
Go off trail and at least 200 feet from any water source. Solid human waste should be buried several inches deep. In high use areas and above timberline, consider using one of the commercial products, for example, a bag with enzymes to break down waste and control odor, to pack out solid human waste. Urine can attract animals, that then dig up and destroy the plants, especially above timberline. So it’s best to pee on rocks if possible.
Keep campfires small or forgo a fire. Know and follow current fire restrictions. Be sure any fire is completely out (cold to the touch) before leaving. Don’t bring wood from another area as that can also bring in pests.
Keep dogs on-leash where required, and under control everywhere. Don’t let dogs chase wildlife, which stresses the animal. Pack out dog waste, which can spread diseases.
Watch wildlife from a distance. Don’t feed wildlife. Keep a clean camp. Store food and trash away from camp, secure from bears and other animals. If animals associate people with easy food, it is bad for the animals and bad for the people.
Trails or areas may be closed seasonally, for restoration and recovery, to protect wildlife, or for some other reason. Don’t create or follow non-system (social, rogue) trails, as this can disturb wildlife, trample plants, and lead to erosion. Respect private property; don’t trespass. Some trails go through private property with permission from the owner, and bad behavior could cause the property owner to restrict or prohibit access.
Horses have the right of way because they can be unpredictable. Stepping to the downhill side, if practical, is recommended because predators tend to attack from above, so people on the uphill side can make horses nervous. Everyone stepping to the same side makes passage for the horses easier. Some horses do not recognize hikers, especially backpackers with large packs, as human, so talking, for example, a friendly greeting to the rider, can help a horse recognize a human and be less nervous.
Bikers yield to hikers and horses. Bikers should always ride in control and be able to stop, especially where curves or vegetation make sight lines short.
While strictly speaking, bikers should yield to hikers, often it’s easier for the hiker to move aside. This is especially true when bikers are going uphill.
Generally, downhill traffic yields to uphill traffic, though uphill hikers might appreciate a chance to stop for a moment.
Trail users should stay aware of others overtaking them and allow faster people to pass.
Trails have official rules of who yields to whom, but if everyone tries to be more helpful than required, things will go more smoothly. We’re all out there to enjoy nature, get some exercise, and have fun.