Nepal's only international airport is Kathmandu's Tribhuvan Airport. If you want to see the mountains as you fly into Kathmandu, make sure you sit on the right-hand side of the plane. The classic overland routes between Nepal and India are still popular.
Buses are usually the quickest and easiest form of transport between Nepal and India.
A trickle of travellers enter Nepal at the Mahendrenagar-Banbassa border crossing in the extreme west of Nepal, which is handy for travellers coming overland from Delhi who do not want to visit Varanasi. The crossing between Nepal and Tibet via Kodari is open to organised groups but not to individual travellers heading north. Be prepared with alternative plans if you're thinking about using this route, because landslides regularly make it impassable during the monsoon. It's advisable to book domestic flights a week in advance and keep re-confirming your ticket just to make sure you don't slip off the passenger list if the flight is full. Airlines only accept payment in hard currency from visitors.