The documents required by you when you apply for a Nepal visa on arrival are as follow:
You need to carry your original passport which should be valid for at least another six months and it should have at least two blank visa pages.
You should have several photocopies of the personal information page of your passport.
You have to take a latest passport size picture of yourself ( measuring 1.5inch x 1.5inch with a plain white background) You should take more than 2 copies of your photographs with you as it might be needed to extend your visa , when paying the fees or for other purposes that may arise in an emergency.
The completed and signed Nepal visa application form.
Proof of your stay at the place in Nepal (proof of hotel reservation tickets or an address as asked by you)
Valid and confirmed return flight tickets
Proof of enough financial means to stay in the country or proof of bank statements from the last 3 months.
Proof of paid Nepal visa fees.
Important Note:- The documents asked at the time of your arrival might change and it will depend on the purpose of your travel to the country. So it will be better to fill the online application form before you travel to Nepal because after completing the form you will receive the exact documents you must have with you before you stand in front of immigration officers.