Gorkhi-Terelj National Park
It's a rather mountainous area with a beautiful river, forests and some interestingly shaped rocks.
Flora and fauna
There is a bus from Ulaanbaatar to the park. It leaves from a city bus stop on the south side of Peace Avenue, opposite of the big 'Narantul Hotel', 150 metres East of the junction with Ondor Gegeen Zanabazaryn Gudamj, which is the road that runs south from Gandan Khiid. The bus stand has a large "e-ticket.mn" sign on it. GPS decimal coordinates N 47.91545 E106.89782. (in Sept 2010)
Buses leave at 11:00am and 16:00pm, and take 2 1/2 hours to get to Terelj village. Coming back, they leave Terelj village at the uncivilised hours of 08:00am and 19:00pm. The bus costs 2300 MNT one way (as of July 2009)
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 MNT payable at the national park gate, 6km from the turnoff for the park road.
Trekking, Horse riding.
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.