Sometimes when one travels to a location for the first time, a person doesn’t pack certain items since they’re not the types of things the person uses on a daily basis or that they bring with them on other vacations.
 
Many travelers to Yosemite National Park responded to a question in the Yosemite forum with suggestions of what to pack for a trip to Yosemite. The suggestions have been organized into a resource for other YNP visitors. Note that some of the suggestions are in more than one category.

Since everyone has their own opinion of what to bring, it would probably be most useful to others if you added your version of an item rather than edited an existing version. (For example, rather than change “signal mirror” to “compass with signal mirror,” keep the “signal mirror” entry and add your “compass with signal mirror” entry.)

Planning  
  • Real-time advice from a park ranger about trail conditions - if the station is closed check the board by entrance station or ask at one the lodges
  • An official weather forecast for each day you will be there (thunderstorms or snow in off summer months?) - lodges will note this by the front desk
  • USGS trail map and guide, preferably with elevation profiles
  • Driving directions to and from the park
  • Maps for inside the park - if the park ranger station is open, you can get a visitor guide with major routes on it
  • Local maps for outside the park
  • Stargazing guide or map
  • Your itinerary  
  • Yosemite National Park Road Guide - published by the National Geographic Society (optional) and there are other guides such as Yosemite National Park  - Lonely Planet.
  • Trails Illustrated Map of YNP - published by the National Geographic Society
  • Yosemite Guide - you can get a copy in the park or download and print a copy before you leave home
  • America the Beautiful Interagency Pass - The pass is optional as you can also pay for as you can also pay Yosemite Fees instead.
Clothing
  • Poncho or hooded rain jacket - for waterfall trails or just in case it rains (optional if summer and not doing falls trails)
  • Convertible cargo pants - pants with zip-off legs and a lot of pockets
  • Cargo shorts - shorts with a lot of pockets
  • Clothing made of quick-drying material (polyester blends or 'wicking' fabric) rather than cotton
  • Sun hat or visor
  • Comfortable, but sensible, shoes, if you're going to be walking any distance. Yosemite's trails won't be much fun in flip flops!
  • Extra pair of shoes
  • Jacket or sweater - for evening 
  • Resort casual if you are planning to go to dinner at the Ahwahnee Restaurant.
  • Rain gear (optional - check the 10 day weather forecast in advance)

Food and Drink

For all

Water LOTS! (for hikers a bladder type water bag works well to keep your hands free for hiking)

For others who want to picnic, hike, etc. some other suggstions are:

  • Picnic supplies
  • Water purification system
  • Personal water purification / filtration sports bottles
  • Fruit (oranges, etc.)
  • Power bars
  • Granola bars
  • Nuts
  • Lots of snacks    
Things to Bring if Hiking
  • Real-time advice from a park ranger about trail conditions
  • USGS trail map and guide, preferably with elevation profiles
  • Basic first aid kit, including an ACE bandage and moleskin for blisters
  • Bug spray - more than you think you will ever need
  • Insect sting kit - if you were bitten in places that the bug spray didn’t cover - or if you’re allergic to insect bites
  • Flashlight and extra batteries
  • Whistle
  • Signal mirror
  • More energy/protein snacks than you think you will need - in case you are out longer than you anticipate
  • Space blanket - has many uses in an emergency and is light weight
  • Matches and some tinder in a DRY container - in case you have to hole up
  • Toilet paper and a Ziplock (plastic) bag to carry it out!!
  • Swiss Army knife / multitool 
  • Hiking stick or poles 
  • Compass with signal mirror
  • Yosemite National Park Road Guide - published by the National Geographic Society
  • Trails Illustrated Map of YNP - published by the National Geographic Society
  • Gloves with a good grip on the palm and fingers if you plan to climb Half Dome using the cables; you can't be sure that there will be suitable gloves in your size at the base of the cables.
  • Sunglasses
  • Alcohol-based, quick-drying disinfecting hand cleaning liquid
Other
  • Bug spray - in the springs and early summer, more than you think you will ever need.
  • Insect sting kit - if you were bitten in places that the bug spray didn’t cover
  • Flashlight and extra batteries
  • Towel
  • Sunglasses
  • Binoculars (several people suggested this)
  • Camera - with extra film or memory cards and extra batteries or a way to charge the one you have 
  • Day pack
  • Sunscreen
  • Baby wipes - they're cheap and come in handy for cooling off and cleaning up on the trail
  • Moist towelettes - similar to baby wipes
  • Multi-socket electrical extension cord - in case there’s only one electrical outlet in your room / tent cabin 
  • Dramamine - for motion sickness
  • Lip emollient (Chapstick, Blistex, etc.)
  • Tent - if you want to camp
Things to Bring if Driving to YNP from Home
  • Comfortable chair and / or great blanket - to just go out and enjoy Yosemite meadows
  • Your own bikes - beats waiting around for buses
  • Your own padlock(s) for bearproof food locker  - otherwise, people are opening and closing (or not) your locker to check for space all night long!   Note: if you are leaving food at a trailhead then you are not allowed to use a lock; it will be cut off by rangers.  This suggestion about a lock only applies to folks who have a "private" bear food locker at their campsite or lodging. 
  • Your own down pillow  (optional)
  • A telescope if you will be in the high country

Things To Bring If Camping

  • Tent
  • Small stove
  • Sleeping bag - suitable for cold weather
  • Plenty of food
  • Sunscreen
  • Bicycle (Valley only)
  • Lip emollient (Chapstick, Blistex, etc.)
  • Toothbrush
  • Toothpaste
  • Deodorant
  • Pillow
  • Pocket knife
  • Long pants
  • shorts
  • t-shirts
  • long sleeve  shirts
  • hiking boots
  • good hiking socks
  • sweater/jacket
  • camera(s)
  • water bottle - refillable 32oz. is good
  • watch
  • snack items
  • The new 3-4 oz. alcohol-based, quick-drying disinfecting hand cleaning liquid