You can fly between Delhi and Kathmandu for about $150 with RNAC or Air India. The actual flight time, not counting the endless delays and cancellations, is only a little more than an hour. However, note that Delhi-Kathmandu-Delhi flight is very busy and without proper reservations (or proper strings to pull) can be booked weeks in advance. Make your reservations and buy your ticket well in advance.

Alternatively, you can travel overland to Nepal from India. Buses are usually the quickest and easiest form of transport for this. There are three main crossing points: Sunauli-Bhairawa, Birganj-Raxaul and Kakarbhitta-Silguri. The Sunauli border crossing is the best one from Varanasi, the Birgunj crossing is the easiest from Calcutta; and Kakarbhitta is the best crossing from Darjeeling. These trip can be quite long and stressful, both in terms of time (it takes about two days and nights) and what you may go through during the trip (with tickets, safety, weather, border harassment etc). Not recommended for those people who want to have carefree travelling.

If you plan to enter Nepal in a car, make sure you have a carnet de passage en douanes. These are required to exempt you from customs duty for three months. You may also be required to pay a fee for each day that your car is in Nepal. As in India, in Nepal, vehicles are driven on the left side of the road.