• Hiking Shoes/Boots - much of the park's outdoor pursuits involves lots of walking and hiking on trails. This allows you to confidently walk through rugged uneven terrain where footing can be uneven or steep. It will also limit your likelihood of twisting an ankle or knee, which can be especially dangerous if you're deep in nature and help is far away. You can downgrade to running shoes or some other comfortable athletic shoe if you're only limiting yourself to the flat walks of the Upper Geyser Basin or developed boardwalks in the Canyon area, but they're not optimal for going on any of the harder trails outside these developed paths.
• Hat - don't take for granted the dangers of UV radiation. A hat will at least keep your scalp from getting severely burned (especially considering how easy it is to get sunburned in high elevation). If you're wearing a broad-rimmed hiking hat, it could also help protect your neck, ears, and face.
• Sunscreen - again, given the sun's harmful UV rays, it's a good idea to protect other exposed parts of your skin from sunburn.
• Sunglasses - prevents cataracts or other harmful effects of prolonged exposure to the sun's ultraviolet rays on your eyes
• Bug Repellant - this can be controversial because the chemicals contained in these things can be harmful to the environment. However, getting eaten alive by mosquitos is not desirable either (especially if there's always that potential of getting some mosquito-born disease). In any case, the most effective repellants contain DEET, but they're smelly and toxic if used excessively. The better way to go environmentally, though not necessarily a comfortable one, is to cover your skin with long sleeves and pants. For Yellowstone, mosquitoes are generally around from late Spring through to early August as they breed in pools and meadows formed from the melting of the snowpack.
• Layered Clothing - given the fact that park's nestled deep in the majestic Rocky Mountains, you'll be in a high elevation area with variable weather, and this can mean the difference between life and death. For example, temperatures swings can easily go from the 80s to 0 degrees F. Don't underestimate the dangers of hypothermia, altitude sickness, and to a lesser extent heat stroke. Preparing yourself with multiple layers that you can peel or add on as needed is a must.
• Rain Coat - this will keep you and your equipment dry for the most part (with a rain poncho covering the pack in addition to the body). This can happen if you're hiking and it's stormy or you're caught in a downpour from an afternoon thunderstorm. Rain gear will at least keep you sane under such conditions (not to mention prevent your camera or other valuable electronics from getting waterlogged).
• First-Aid Kit - you'll never know if you need to stop bleeding or remove a tick lodged in a fatty part of your body. It's always a good idea to carry this around in your pack to keep an annoying situation from become life threatening.

Some other things worth bringing to enhance your experience include...

• Hiking Sticks/Trekking Poles - this is useful for longer and more involved hiking excursions. They provide you 1 or 2 extra "legs" to maintain your balance on stream crossings or alleviate shock to your knees if you're carrying a pack
• Bear Spray - this is basically like a can of mace except it's marketed for self-defense against a grizzly bear attack. I did carry one of these around while hiking in Yellowstone, but you have to realize that this is really a last line of defense as you really don't have much of a chance against a bear attack. False sense of security perhaps? Maybe, but at least you'll have something to fight back with if it does come to that.
• Bear Bells - the point of these noisy jingly things is to let a grizzly bear know you're coming. The problem is that if it's windy or the sound doesn't travel well (due to canyon or thick foliage), then this won't be much help. Warning: these can be quite annoying! Perhaps keeping a loud conversation going in a larger group of hikers is a better way to go to lessen the odds of surprising a grizzly (especially mother and cubs) and hence stirring an attack.
• Bear Cannister - even though bears out here haven't figured out how to break into cars or steal your pack, this doesn't mean you can't use the extra precaution. Perhaps the ecosystems are healthier here than in California where they haven't acclimated to human food yet. Just something to consider if you don't want to hang your sacks of scented items or not want to chance your car getting broken into.
• Lots of Memory or Film or Portable Hard Drive - the first and third items are for digital photographers. In any case, you'll be taking heaps of photos and you'll want to make sure you can bring all your photos home

When it comes to nature, keep in mind that nature is inherently dangerous and unforgiving. But in order to enjoy what Yellowstone has to offer, you'll have to respect nature and be cognizant of these dangers. Below is a list of things you need to be aware of to stay safe in the outdoors...

• Hypothermia - as mentioned earlier, this is a silent killer that can catch those who got lost, debilitated, or were unprepared. Given the fact that waterfalls tend to be located in forests and mountains (i.e. the type of conditions you'll see in Yellowstone), you can bet night time temperatures can easily dip to freezing or colder. If you don't have adequate clothing, you will indeed freeze to death. So come prepared with layers of clothing, check the weather, and study where you're going so you don't get lost. Do that and you'll be fine.
• Drop-offs - waterfalls tend to be located where there are canyons, ravines, cliffs, gorges, etc. and Yellowstone is full of these steep areas. That means, drop-offs are real hazards, and a fall from great heights can not only break bones, but they'll kill (which is usually the case). Your best defence against this is to stay away from cliff edges (especially if there are no railings) as you don't know how stable the ground beneath you can be. Also be wary of off-trail scrambles down cliffs as you may find yourself unable to get back up! Be prepared to ask yourself whether that better photograph is worth it and walk away.

• Thermal Features - since Yellowstone sits in the caldera of an active supervolcano, the earth's crust is thin and prone to instability. You will undoubtedly appreciate this when strolling along the boardwalks of the park's many geyser basins. In fact, sometimes walks get re-routed or closed as geothermal features can change their behavior over time. Bottom line is to stay away from thermal pools, springs, and geysers. People have been boiled and burned to death or have been gassed to death due to noxious fumes.
  
  
• Altitude Sickness - this is a problem, especially in the high elevation parts (such as Canyon, Dunraven Pass, etc.). If you know you're going to do a hike in high elevations, keep in mind the air is thinner and you'll be taking in less oxygen (that's why it's easier to be short of breath). You can try to counteract this by acclimating with a night's stay before your long hike or backpack. But succumbing to altitude sickness is not pleasant as you typically get nauseated, light-headed, or disoriented.
• Getting lost - always stay on the trails and heed the signs - for both your safety and protect the fragile environment. Bring a topo map as well as a compass with you if you're on a more involved hike where it's easier to lose the trail.
• Heat Stroke/Dehydration - even though Yellowstone is in a mountain range, it can easily get hot (especially in the summer) and not having enough water to drink is typically the number one way for you to overheat. In fact, getting a heat stroke can often lead to deaths so don't underestimate this threat. Preparation and lots of water is your best defense against this.
• Wildlife - Yellowstone is home to grizzly bears, wolves, coyotes, bison, elk, moose, and deer. Grizzly Bears are well known for documented cases of bear attacks against humans. Realize that you're in bear habitat and respect it as such. Try not to surprise them and leave them a wide berth if you do see such a bear. While bison, moose, and elk are thought of as docile grazers, they can attack humans and kill. Bison are especially dangerous given their size and their tendency to get agitated. The elk and moose rut is particularly a time when these organisms get violent. Definitely don't even think about trying to feed wildlife!
• Insects - there can be insects that can be harmful. One potentially dangerous threat is that of ticks. This can especially occur if you're in a primitive hike where the trails tend to narrow and force you to brush up against side brush. Many of them can carry diseases (extracted from deer, rats, squirrels, etc.) such as Rocky Mountain Spotted Fever, Lyme Disease, and other bacteria-based diseases. I personally had a tick plunge itself right into my thigh and fortunately didn't contract an illness. If you do happen to find a tick, try to carefully remove it with tweezers from its head. If the tick breaks with its head still in your body, you can easily get an infection. It's under these circumstances (among others) where a first-aid kit is handy to bring with you.
• Streams and Rivers - this can be a real hazard in the spring thaw months (going into early summer) when waterfalls are raging but so are the streams and rivers. Since many backcountry hiking trails involve unbridged river and stream crossings, you'll generally want to wait until middle to late summer to partake in such hikes anyways. But if you do find yourself having to make a deep crossing (don't even think about crossing anything waist-deep or higher), wide and shallow crossings are more preferable than short but deeper crossings. Also, unbuckle a heavy backpack just in case you fall so the pack won't keep you from getting up and causing you to drown.

Most people tend to spend a week or so on a typical vacation. We were no different on our Yellowstone trip. Though the longer the trip, the less rushed it will be and the more flexibility you will have. In any case, our 8-day trip was plenty to see all the Grand Loop attractions and go on a few longer hikes (such as Osprey Falls) as well as a day and a half in the Grand Tetons.

• litter
• irresponsible off-trail shortcuts
• people feeding wildlife
• graffiti on rocks or trees