Set in stunning natural surroundings, Vat Phou, the ancient Khmer temple complex, listed as a UNESCO World Heritage site, is the main attraction in the area, but there is much more to be discovered. It is worth staying in the charming town of Champasak where you can discover French colonial buildings, Buddhist temples, or simply enjoy an evening in the Shadow Puppet Theatre. You could also take a relaxing massage at the Spa or take a boat to the Mekong island of Don Daeng to relax in its tranquil environment and discover local village life on a bicycle.
Pakse is the transport hub of the region, with an airport, bus service and limited boat access off the Mekong. For Champasak and the South, there are minivans (110,000 kip return to Si Pan Don) bookable from most hotels and tour agents in Pakse town centre. These are nearly always packed with irritating 20 somethings usually wearing Beer Loa singlets and spotting scars from scooter accidents, also don't expect any leg room.
From Pakse southern bus station, take a songthaew (starting 9am until 11 am; could be very crowded, 20,000 Kip) to Champasak (~2h) or Than bhan khop (2h15), near the crossing to Wat Phu. In the latter case, you have about 30min walk to Wat Phu entrance.
Most places in Champasak, including Wat Phou, can be visited as day trips from Pakse.
Thao-Tao ruins of medium size temple 1km from the previous one, difficult access on dirt road and forest. To Mo ruins on the other side of the Mekong. Lingaparvata sacred mountain (1416mt.). The top has a natural stone lingam of 10mt which can be seen from the bottom of Wat Phu. Impossible to reach the top, maybe with 2-3 days trek.
All these sites have the status of UNESCO world heritage.