Langtang National Park
This region is full of mountains: Langtang Lirung (7246m), Gang Chhenpo (6388m), Naya Kangri (5846m) and Dorje Lakpa (6966m). The Gosainkunda lake (4300m)and the Dorje Lakpa range (6988m) bisect the park from east–west to south–east. The summit of Langtang Lirung (7245m) is the highest point in the park.
Flora and fauna
Langtang and Helambu regions is the second largest conserved National Park of Nepal enriched by 1000 species of flora and 150 different species of birds and many wild animals such as tigers, bears, deer, and wild sheep.
From October to November and from April to May, days are warm and sunny, and nights cool. In winter from December to March, days are clear and mild but nights near freezing.
Public buses are available to Syabru Bensi, where the most known treks start. There is a local and a deluxe bus, that is little more expensive. Be aware that the deluxe bus may have LCD and sound system with most horrible Nepali Pop Music playing. Don't sit in the back, the road is very bumpy (Dec '18). You need to book tickets before, depending on season starting 1 day before the ride. For the way back, you'd get a ticket in Syabru Bensi for the next day (in Dec '18) A jeep to Syabru Bensi will cost about $200.
Entry Fee Foreigner 3390 NPR (Dec. '17)
Individual Trekking (it's exciting!): It is common and definetly possible to go trekking in Langtang without a guide. You may need a guide for crossing high passes in wintertime. Prepare your individual trekking: The more days you have, the better your trek will be. Of course, you can do a short trip to Langtang Valley and back, but you will miss most of the exciting things. There are many hot tips you'll probably know about while trekking (Cerko Ri, making friends with people from local villages etc) Before Kathmandu: Think about the time you'd like to spend in the mountains. You may not decide for a number of days since your route is not concrete yet, but you may think about it (maximum days, approximately etc..) Download a trekking map and start reading itineraries, travel blogs etc. for orientation and for getting to know the endless and exotic possibilities for trekking. The three main treks: Langtang Valley and Kyanjin Gompa, Gosainkunda Lake and the others, Tamang Heritage Trek. In Kathmandu: You may want to get a good trekking map in one of the trekking travel agents shop in Thamel. Otherwise, we found an old but okay.. trekking map Syabru Bensi, too. If you are into trying new things, get a compass. Get your bus ticket to Dhunche or Syabru Bensi. (We got it via travel agent shop) You may not even need a bus if you start trekking from Kathmandu. For the individual Langtang/Kyanjin Trekking (Dec. '17) we didn't need a TIMS card. The entry fee we paid on the way to Syabru Bensi(the bus makes a stop for foreigners) In Langtang: Enjoy your trip and don't forget about protecting your environment! Other tips: Start early (between 7-9 am) Depending on your physical (dis-)abilities you may decide for yourself how many hours you can walk each day, f.ex. I walked all the way from Kyanjin to Bamboo in one day and it way okay. Another guy walked all the way Syabru Bensi to Kyanjin Gompa in two days (possible if you really don't have problems with high altitudes).
On Equipment: If you have a sleeping bag that is good and small and light, bring it. It's more comfy and for higher altitude lodging and camping you'll need it. Otherwise, don't bring it (if you want to have less weight in your bag). There a more than enough blankets (Dec.'17) and if you have warm clothes (warm and light and musts for Trekking in Himalaya!) you will be fine.
On Food: Bring biscuits and whatever and how much you like to eat (and not buy from restaurant) from Kathmandu. Stuff is way more expensive up there. You may want to read about Trekking food. You can buy biscuit stuff in the regular general store (they have a fixed price written on the item). You can buy cereals and more things in (local!) supermarkets. Fruits on the streets.
It is a teahouse trek. This means that you'll find a lot of restaurants mostly integrated into guesthouses/lodges etc. There are also some bakeries. There is no (obvious) way to eat 'uncommercially'. However not every guesthouse is like the others. In my experience, some people were nice and personal, some more service-orientated. Although they serve you, don't hesitate to talk to them if you like! Prices: They are not depending on season but on height. Generally the food is very expensive and food menus are quite 'western' (Snickers Momo, Pancakes..) although you'll find 'traditional' food, too. (Momos, Dal Bhat w/curry etc.) Dal Bhat price may vary 400-700 NPR. If you are on a budget you should explain your situation beforehand. Food prices are negtioable but be aware about how your food went uphill. Also, inform them if you want/need to eat your food you bought before and won't eat a breakfast f.ex. Normally, they will welcome you very warmly and understand your situation perfectly
This is where most people sleep, especially around the main treks. During off-season time (in my case Dec. '17) sleeping is free of cost, although you should order at least something in the integrated restaurant. WiFi is rare, hot shower you may ask for (mostly with extra fee but some have it for free). Don't always take the first Lodge that you'll find (or they find you) in the small villages since they have a geographical financial advantage and there are quite many (may better) ones a few meters afterwards.
For the Langtang Trek, you can do it very less since there are not many small plateaus around the trek. But there are some and you may camp there if you like. Camping for longer Treks (where there are no tea houses) or short treks to peaks etc., I dont't know about.