Where the road to Wat Phu turns away from the Mekong, the remains of the Champasak's ancient city ('Muang Kao') begin. Founded in the mid-5th century by King Devanika and lasting for at least three centuries, there are pre-Angkorian walls (both earthen and brick) and other mostly still-buried remnants spread across the nearly 4-sq-km site. But, though you pass right through the heart of it on your way to Wat Phu, you can't see anything ancient from the road.
There's a map of the many ruins just south of the bridge on the site's river side, but you'll need a guide if you want to explore it properly. And probably only serious history buffs will get anything out of the experience.