Here is a list of recommended things to bring when you visit Grand Teton National Park and Yellowstone National Park: (Provided by Xanterra)
WHAT TO BRING FOR YOUR YELLOWSTONE VISIT
When visiting Yellowstone, it is imperative that you get out the map and plan your route, because the park is huge – 2.2 million acres, in fact. Driving across the park can take several hours because of the sheer distances involved as well as the possibility of wildlife sightings creating the occasional “bear jam.” The following is a list of other items we suggest you pack to ensure a comfortable stay in Yellowstone National Park during summer or winter.
Summer Visit to Yellowstone
Insulating underwear: Something that has the ability to wick moisture away from the body.
Mid-weight insulating layer: A light synthetic fleece or wool shirt/pullover.
Heavyweight insulating layer: Wool, down or heavy-weight fleece jacket (especially May, June, September, and October.)
Waterproof and windproof outer layer: Lightweight, and breathable if possible. Both jacket and pants are recommended.
Two Hats: A brimmed one for sun protection and possibly for rain and an insulating one for cool weather (May, June, September, and October.)
Gloves: Lightweight glove liners, and possibly even a pair of heavier wool/fleece gloves or mittens if you are visiting in May, June, September or October.
Socks: Light wool or synthetic liner sock with a heavier wool or synthetic outer sock, the inner-outer combination helps prevent blistering, wicks moisture and reduces friction. Cotton socks are not recommended.
Footwear: Hiking boots or good or athletic shoes depending on what you plan on doing. Any shoes must provide stability, traction and comfort. Sandals, athletic shoes or other leisure footwear can be used for relaxing or easy walking.
Daypack with enough capacity to carry extra clothes, water, lunch, camera, binoculars, field guides, etc.
Water bottle: One quart/liter is the minimum recommended. Camelback or similar hydration systems work well.
Thermos for hot liquids
Sunscreen/lip protector: Sun at high altitude can burn unprotected skin quickly.
Insect repellent (summer)
Camera, memory/film and batteries
Binoculars and/or spotting scope and tripod
Non-electric alarm clock: Recommended if you want to be sure to be someplace on time.
Any prescription medications