Sunauli is the traditional name given to both sides of this India/Nepal border crossing, 70 kilometers north of Gorakpur and 3 kilometers south of Bhairahawa. Technically the Indian side is "Sunauli" and Nepal side is Belahiya. Both are small, congested, dusty and dirty towns and the quicker you can get on to Bhairahawa (again only three kilometers north)/elsewhere, the better.
Sunauli is not a place to linger. But it is an important border crossing/trade point, therefore the traffic and congestion. There are a few travel offices, currency exchange facilities on the Nepal side, and zero sights - unless you count long lines of Indian big-rigs. If you are having Indian Currency and going to Nepal, no need to worry about exchanging money to Nepali Currency. Shops/Hotels/Bus in Nepal easily accept Indian Currencies of denominations up to Rs 100.
Going from India to Nepal, Nepalese visas are available on arrival. Going from Nepal to India? Hope you have visa already as no visa's are available on arrival to India. And neither immigration offices are overly helpful.
Indian "package" tour buses, mostly pilgrims to Lumbini and Kathmandu, do cross the border and continue on after immigration. The farang / backpacker "package" tour buses normally do not and you will sadly and most likely be booked overnight in a cheap, not so nice hotel in Sunauli for your connection "connecting service " to Kathmandu. If you have only booked to the border you can cross (on foot) and find buses on either side up to 6PM - 7 PM. Intercity buses depart on both sides usually twice a day - dusk and dawn(from 4 AM) . Delhi, Gorakpur, Sunauli, India/Belahiya, Nepal. 24 - 30 hours. Try not to cross border in the night, Security persons on both sides are not co-operative and hotels on the Nepali sides will be closed, you have to rely on a guide.
Via Delhi, Gorakpur, Sunauli, India/Belahiya, Nepal. 24 - 30 hours.
Via Kathmandu, Narayangarh/Bharatpur (var. Narayanghat), Butwal, Bhairahawa, Sunauli. 7 - 10 hours.
Via Pokhara, Palpa/Tansen, Butwal, Bhairahawa (potholed, windy road, but beautiful views) 5 - 6 hours. Also Pokhara, Narayangarh/Bharatpur/Butwal, Bhairahawa, Sunauli (better road conditions). 5 -6 hours.
Via Gulmi-Tamghas, Palpa/Tansen, Butwal, Bhairahawa (potholed, windy road, but beautiful views) 4 - 5 hours.
Via Delhi - Gorakpur. Transfer to narrow gauge track to Nautanwa. Transfer to taxi/bus/rickshaw to Sunauli, India (about 5 kilometers). Cross border to Belahiya and Nepal's Immigration. Total time approximately 12 - 18 hours.
Via Kolkata, Patna, Gorakpur. Transfer to narrow gauge track to Nautanwa. Transfer to taxi/bus/rickshaw to Sunauli, India (approximately 5 kilometers). Cross border to Belahiya, and Nepal's Immigration. Total time approximately 10 - 12 hours.
To Gorakhpur by jeep, around INR 100, 2-3 hours.
Rickshaw's, buses, taxis all available. If you are stuck overnight here do not hesitate to take a 10 minute rickshaw up north for wonderful restaurants in Bhairahawa.
Options for food on the Indian side of the border are very limited and of questionable standards (with the exception of the pilgrim's hotel). Eat on the Nepal side and/or pack extra food if possible. Look for nice little outdoor daba/tandori restaurant run by Lucky Singh just 30 meters north of the border on the east side. Questionable atmosphere but great food! They even have A/C facilties (20 NRs extra) in the hot season.
Most of the accommodations are found on the Nepal side, though there is a Government of India run hotel for pilgrims on the Indian side. Look for the large white building about .5 kiilometers south of the border on the main road. Modern rooms with bathrooms, TVs, and A/C for 1200IRs. None are recommended for women travelling alone. . Cheaper rooms usually go early in the day.
Getting out from Sunauli is easy during daytime, but harder in night. So plan your journey according to that.