Thanks, jmkrsna, for the prudent warning. This is a real pet-peeve of mine. If you come to Nepal and want to trek, it is very important to provide a job for someone here. Although they would like an hours pay for a days work, they will often settle for 500 NRs. for a full days' pay. That's only 5 euro/$6.
Please remember to provide a job for someone while you are here. It isn't just good karma, but it will also add to your enjoyment and make your time safer. No one deserves to be robbed or worse, but there is a resentment building that I've noticed here.
How can they add to your enjoyment? For example, I was walking along the trail when my guide pointed out a plant as she tried to tell me not to touch it. Well, she was too late and I got some poison in my finger that made it hurt until the next evening. Imagine if I had to squat in the woods! They know their country and will protect you from harm.
And if you want to go to the UNESCO sites please, please pay the admission. If you bring a copy of your visa and two passport size pictures you can go back for as long as your visa is good. There are also many free temple sites you can enjoy.
WARNING: If you get caught in the conservation areas without a permit you can get deported and have that stamped on your passport. If that happens you will have to go back to your home country even if you have a ticket for an around the world trip. The fine is pretty huge, also. If you did manage somehow to go on without returning home you will be met with a very bad welcome from each country you enter because they can see you got deported. I know someone who got caught and it was a big fine and deportation, so don't do it. There are many free trekking trails, so don't think you can't enjoy Nepal.
Nepal has something for everyone. Check out my blog at FrugalTravelsNepal.Blogspot.com for a lot of suggestions on making your trip to Nepal the most amazing vacation ever-on a budget. I write about things to do and see in-between major activities, great shopping values, how to stay safe here, etc.