Kathmandu is subtropical, so it is warm (70-80) during the day and cooler (50-60) at night. Though in this culture it is still customary to keep legs covered above the knees as well as the shoulders, so pack warm weather clothing that isn't revealing.
1 pair of shorts or skirt (avoid anything too short)
1-2 tanks or t-shirts
Shawl (if you need to cover yourself in temples)
Sandals or sneakers (comfortable to talk around the city in)
Flip flops for shower (if you prefer)
It is cold but if the sun is out and you are moving it is comfortable. Know your boundaries in cold weather, if you tend to be cold then prepare accordingly! Make the most of what you bring, try to bring things that can be used multiple times to keep the pack light. Two keys: you want to be warm and comfortable. My suggestion is to bring one to two luxuries that will make you really happy at the end of a long hiking day, BUT please be aware that weight & size matters! You are going to be wearing clothes repetitively so pick your favorites and make it work.
Lightweight hiking pants (1-2)
Heavier-weight hiking pants (1-2)
Base layers- long underwear, tank tops/t-shirt, long sleeve
Mid layer- fleece and/or light down
Hard Shell jacket
1-2 winter hats or headbands
Baseball cap or hat with visor
Polarized sunglasses or glacier glasses
Water-proof, breathable jacket and pants in case of rain (Gore-Tex is ideal)
4-5 pairs of medium weight hiking socks
1-2 pairs of heavy-weight socks
Hiking boots or hiking sneakers (light to mid weight)
1-2 buffs or scarves
A pair of shoes to wear when you are hanging out, I would suggest something comfortable like Crocs or Uggs
Yoga mat- ideally something that isn't too heavy (they make some great, lightweight travel mats!)
1-2 pairs of yoga/lounge pants
Snacks- 1-2 per day max, we are fed three meals a day so these are just supplements for the hike. Don't be stoic and only pack what you think is best, make sure you enjoy eating it too!
Backpack- A good quality large (roughly 3,000 cu. in.) day pack. This can be your carry on for the airplane. It should accommodate all of your camera equipment and some extra clothes etc. that you will need each day for the trekking.
Rain cover for backpack (comes with most backpacks)
2- 1 liter water bottles (I recommend Nalgene, you want the bottles to be lightweight)
Down jacket, sleeping bag, micro spikes and waterproof duffel are provided, though if you prefer you can bring your own
Down booties or heavy-duty socks for sleeping
Toiletries, Medicine Bag & Miscellaneous
Though our guides have an extensive first aid kit it is recommended to bring some things on your own. Nearly all of these items can be bought in Kathmandu (much cheaper) so don't worry about bringing everything.
Activated Charcoal, Pepto Bismol, Tums or any other kind of stomach aid
A high quality multivitamin and probiotic will help your immune system throughout all the travel
Chlorine or Iodine water purifying tablets
Saline Spray (keeps nose hydrated in dry, high altitude)
Toothpaste, brush, shampoo, conditioner, soap, etc. (A small travel container of each should suffice as you likely won't shower every day on the trek)
Handkerchiefs to use in place of tissues
Sunblock for face and lips
Ziplock bags to keep all your items organized and dry
Camera gear (batteries, tripod, etc.)
Solar charger - not necessarily but really nice to have as electricity becomes limited the higher we go
Note: Most of this you can buy [for cheap] in Kathmandu
Documents & Cash
Visa - you get your visa upon landing in Kathmandu
2 Passport photos - 1 for visa, 1 for HHE
Journal & writing utensils
1-2 books, iPad*, iPod* or anything else you like for entertainment. There will be ample time to relax each day.
*Charging/electricity is something you need to pay for at higher elevations, I don't recommend being fully dependent on electronics
REMEMBER you are carrying all of this! Mind the weight of your entertainment
Wallet/Money/Credit/Debit Cards- You can easily change money or take money from an ATM in Kathmandu
What to pack it in
Make sure you can fit everything you need for your trip in a large duffel bag, you may want to bring an additional smaller duffel to hold all of your clothing for Kathmandu with potentially some extra space to hold all of the awesome buys you find in Kathmandu. Use your backpack as a carry-on and it will hold your daily necessities on the trek.
I highly recommend both Travel Insurance and Evacuation Insurance for this trip! It is a big trip and you want to make sure you are taken care of in case of misfortune. Evacuation Insurance will protect you if something happens on the trek and you need a helicopter to get you, this could save you up to $5,000. If you join American Alpine Club you receive this insurance, plus lots of other perks, it's a great deal.