Wat Tham Pha Daen is the best excursion from Sakon Nakhon city. The temple sits in the Phu Phan hills and combines forest shade, carved boulders, garden platforms, and wide views over the city and Nong Han Lake.
It is more elaborate than austere, with modern religious sculpture worked into the rocks and terraces. That makes it popular with Thai visitors, but the setting carries the visit: you get both a temple and a proper sense of the hills behind the city.