There is no serious sightseeing in Golmud itself; the city is here because of its position on the railway and the road south, not for its monuments. What's worth your time is the country around it — the Kunlun mountain pass on the road to Lhasa, the immense Qarhan Salt Lake to the east, and the rail journey out of Golmud itself, which begins the long climb onto the Tibetan plateau.
If you have a half-day before your train, walk the central streets. The Hui Muslim quarter east of the river has small noodle and lamb-skewer shops worth a meal. There is a single underwhelming city park.