Enclosed excavation sites, including one with a partial skeleton of Siamotyrannus isanensis, an early ancestor of Tyrannosaurus rex, can be reached on trails from the Phu Wiang National Park's visitors centre or nearby parking areas. The nearest of the four sites is an easy 300m walk, though the most impressive (Site 9) is the furthest. If you have a high-clearance vehicle you can drive pretty near it. There's some superb scenery along the trails.
The trail that used to complete a loop back to the visitors centre has been closed, so you'll either need to retrace your steps or use the park road to get back. Either way, it's about a 5km round-trip.