• 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 Yosemite Valley, but they're not optimal for going on any of the harder trails outside the 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 California's sunny weather). 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 Yosemite, mosquitoes are generally around from late Spring through to early August.
• Layered Clothing - given the park's high elevation (4,000 ft in Yosemite Valley) and varied climates, this can mean the difference between life and death. For example, temperatures swings can easily go from the 80s to near freezing. Don't underestimate the dangers of hypothermia and/or 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).

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 at 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 Cannister - believe it or not, California black bears are smart and they've figured out that people are sources of food. So if you don't want a bear breaking into your car or running away with your pack, you'll need to store anything with a scent (not just food, but also toothpaste, deodorant, etc.) in one of these bulky cannisters if you're out on a backpacking trip. Besides, these are mandatory to use by law (there are also bear lockers at campsites and parking lots throughout the park if you're not backpacking). The park service rents cannisters out for backpackers, but you could also bring your own.
• 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 Yosemite 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 Yosemite), you can bet night time temperatures can easily dip close to freezing. 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 Yosemite 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.
• Rock Falls/Landslides - the granite landscape of Yosemite lends itself to erosion by rockfall or landslide. It's how you get the vertical cliffs in the first place after ancient glaciers did the initial work. In fact, a rock slide back in 1997 near Curry Village that killed a hiker serves to remind us of just how dangerous and real this danger is. Daring climbers are no strangers to this hazard, too. There's no real defense against this since you are in nature, but there are things you can do to try to limit your exposure to this hazard by avoiding places where they're most likely to occur such as the bases of waterfalls (water tends to erode) or even cliffs.
• Altitude Sickness - this is a problem, especially in the high country (typically over 6,000ft). 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 Yosemite 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 - Yosemite is home to mountain lions, rattlesnakes, and even California black bears. Mountain lions can be a threat especially since their domains have been shrinking throughout the state of California (let alone Yosemite); thus making them desperate for food. Rattlesnakes are actually quite common especially in the drier parts of the park like Hetch Hetchy or even the high country. Thankfully, they normally warn you with their rattle before they strike. However, if you do get bit, they're poisonous venom is toxic enough to kill you without getting treated with antivenin in time. Finally, black bears can be scary (especially if they're acclimated to human food to the point where they'll attack you), but they're normally more scared of you than you are of them. There are other wildlife that can pose hazards to you if you do something stupid such as deer (powerful kicks), squirrels (carrying the black death), etc. Please don't feed the wildlife. If you've hiked the John Muir Trail, you may have noticed how aggressive the squirrels are. They've obviously gotten accustomed to human food and are now demanding it from unsuspecting visitors. Imagine a bear doing the same thing!But feeding them is sure to send them down a path that is detrimental to both humans and bears.
• 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 sping thaw months when waterfalls are raging but so are the streams and rivers. The waters here flow with such force that you'll want to stay away from the river's edge because once you're swept in, you're not getting out! Eventually, you'll plunge over a waterfall to your death, and this happens quite often every year. As you get into the summer months, you can be hiking into the backcountry areas involving the crossing unbridged streams and rivers. In these circumstances, use your hiking stick to gauge depth and do not cross if it has been flooded or you can't see the bottom. Generally waters thigh-deep or higher are very dangerous for crossing. I recalled in one instance where water was flowing over parts of a footbridge beneath Wapama Falls. So it can occur.

Most people tend to spend anywhere from about 4 days to a week on a typical vacation. We tend to make our Yosemite trips quick-hitting trips of around 2-3 days encompassing a long weekend (including the long drive). We've seen some people stay longer than a week, but that's not too common - especially given how little vacation time Americans tend to have.

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