Hiking boots (waterproofed, high top) 
3-5 pairs of hiking socks 
Water shoes for swimming
Rain jacket or poncho (Including a waterproof backpack cover - Sierra squalls can blow up quickly.)
Lightweight jacket or fleece 
Sweater or sweatshirt
Shorts or zip on legs
Long pants 
Long Underwear depending on weather forecast. 
I use a lighter sleeping bag and use long underwear if it is cold.
Swimsuit (bring an old beater because the bottom of the hot tub can rough it up a bit).
Hat with brim 
*Warm hat for early mornings or evenings outdoors
*Gaiters (keep snow, rocks, sticks out of boots)
Thin belt that will not dig in under your pack hip belt.
Backpack 3,000+ in3
large garbage bag to use for waterproof liner or a backpack cover
Tent or Rain Shelter
Sleeping bag (35°) [7, 10]
Water containers (2 qt/liters total) - at least 2 containers so that 1 can be purifying while you can still drink from the other. This could be a bladder and a water bottle or 2 bottles.
(labeled so they don't get mixed up)
Plastic lunch box (for sandwiches, cookies, pickles, salad) 
* Plate [6b] Mesh bag for drying dishes
Water purification tablets will be supplied
Toilet paper and baggie
* Wipes cut into small pieces- use 1 daily
* Nature guides 
* lightwieght Camera, film, charger
* Hiking poles
Moleskin for blisters (It will also be carried in the First Aid kits.)
THE REVISED TEN ESSENTIALS 
It started from a list I did for friends on backpack trips I organized 40 years ago (before polyester, sun block, gortex, ...) and has evolved with tips I got form other Sierra Club leaders (see below), Backpacker magazine (www.backpacker.com/gear/), REI (www.rei.com/expertadvice/camping), Books and hanging out at my local outing supplier (They let me do member recruitment and fund raising at their store once a year). I updated it for lodge trips I started running with day hikes out of Clair Tappan Lodge 7 years ago and again when I started leading intro to backpacking 3 years ago.
Acknowledgments: Thanks to Kathie Fowler, Linda Conklin and Francy Rubin from whom I stole a lot of the information on this list.