On the Mekong River south of Savannakhet is this set of Cham or Khmer ruins (the name means Stone House), built between AD 553 and 700. Apart from a few walls, most of the stones of this pre-Angkorian site now lie in piles of laterite rubble. No carvings remain, the only known lintel having been carted off to Paris.
It's a long haul by public transport and you'd need to be a truly dedicated temple enthusiast to make the trip. Sǎwngthǎew (30,000K, two to three hours, 78km) leave Talat Savan Xai when full, usually in the mid-morning. With your own transport, head south along Rte 13 and turn west at Ban Nong Nokhian, near Km 490, from where it's a dusty 17km to the site. Guided tours are also available from Savannakhet.
