Hiking on the mountain trails in Nepal, or even the more challenging and dangerous city streets of Kathmandu, the right backpack with the right outdoor gear will keep you well prepared and free from fatigue over the long distances. The first consideration is whether your group will be camping alpine style (packing all of your own equipment), whether you will be tea house trekking (where you rely on local lodges each night), or whether you will camp in tents and travel lighter, relying on porters to transport your gear. If you have a lot of your own equipment to pack, you will need a large backpack with a light but sturdy support frame to bear some of the weight on your hips. It's good to balance the load a little high to set the weight on your shoulders, but also to fasten the lower strap just above your hips to catch some weight on your lower back. This applies just as well to a smaller day pack that you'll carry for day hikes where you return to the camp or lodge on the same day. For a smaller load you'll only need to carry the immediate items for the day's hike.
Inside the pack you'll place the more immediate items, like lunch and your clothing layers at the top. Next you prioritize for what else you may want to bring, like batteries and extra water. Accommodate the contingency for wet weather or falling into a stream by waterproofing your pack. Either spray Camp Dry on the pack as a light waterproofing treatment, or line the inside of the pack with a big, green waterproof bag. There is also a backpack cover to keep out the rain (but not as good for falling into streams). Some items, such as water, sunscreen and insect repellent (or salt for the leeches) should be accessible without having to take off and open up the pack. Be sure to to bring a pack that has extra webbing or pockets on the sides to fit Nalgene bottle and extra small supplies. As far as drinking water you may want to use a Camelback that is a flexible plastic water container with a long tube and a bite plug on the end. That way you can hydrate at all times without reaching back to get to the water bottles.