Phu Phan Mountains
Highway 213 south from Sakon Nakhon towards Kalasin crawls over the Phu Phan mountain range, which has some interesting sites on its slopes. Some buses to Kalasin and Mahasarakham will drop off and pick up passengers along the way, but most won't. It's best to use sǒrng•tǎa•ou from Sakon Nakhon's old bus terminal.
Phu Phan Rajaniwet Palace
The grounds of the royal family's Isan home, 14km south of Sakon Nakhon, are open to the public when not in use. It's quite a modest residence compared to some of their other palaces, but the gardens are beautiful and peaceful. Only walking is allowed around the main grounds, but cars can go to the elephant corral.
Phu Phan National Park
This national park remains relatively undeveloped and isolated. It's no surprise that the area once provided cover for the Seri Thai resistance fighters in WWII and People's Liberation Army of Thailand (PLAT) guerrillas in the 1970s. The former used Tham Seri Thai cave as an arsenal and mess hall.