Gorkhi-Terelj National Park (Горхи-Тэрэлж) is in Tuv of Mongolia, 55 km outside of Ulaanbaatar. It's one of Mongolia's top attractions.
Terelj is considered one of the most scenic areas in all of Mongolia and is a short drive outside of the capital. It's a rather mountainous area with a beautiful river, forests and some interestingly shaped rocks. The beautiful, mostly unspoiled natural area is a popular place to go for a picnic or spend the weekend for Ulaanbaatar's residents. Due to its popularity, there are many tourist camps and activities inside the park.
There are 2 buses per day from Ulaanbaatar (UB) to the park (July 2018).
Departing UB at 12:00 and 16:00, they are not conveniently timed for short visits to the park, but are the most inexpensive way of getting there.
The bus, Route XO4, has a sign for "тэрэлж" and leaves from a regular city bus stop on the south side of Peace Avenue, opposite and 150m east of the 'Narantuul Hotel' and about 300 meters to the east of the junction with Ondor Gegeen Zanabazaryn. The bus waits at the east end of the bus stop.
Buses take about 2 hours to the south end of the park, 2.5 hours to Turtle Rock and 3 hours to get to Terelj village, which is the final stop.
Coming back, they leave Terelj village at 08:00 and 16:00. The bus costs 2500 ₮ one way (all prices here June 2018).
Alternatively to get back, take the microbus from Terelj village to its last stop Nalaikh town (every two hours, 10:00, 12:00, 14:00, etc., 2000 ₮). There buses leave for UB every 10 minutes (900 ₮).
The road is paved all the way, fine for cars. Lots of UB residents come for the weekend, so you can probably find a ride back to UB pretty easily. Offer to help pay for petrol.
3000 ₮ payable at the national park gate, 6 km from the turnoff for the park road. Depending on your mode of transport and the time of day/year there may be no one to collect the fee and/or the bus may not stop at the gate.
Trekking, horse riding, rafting, and golf during summer. Cross-country skiing during winter.
There are a couple of standard shops in Terelj village if you want to stock up on bottled water, biscuits, vodka, etc. Tourist camps sell stuff as well but will have less variety and higher prices.