Go river tubing down the Nam Song

Rock climb the limestone cliffs with a guide ($25-35 half day)

Take a hot air balloon ride over the karsts at sunrise ($90-100)

Zipline at the Vang Vieng Challenge or similar adventure parks ($15-25)

Swim at Blue Lagoon 1, 2, or 3 (entry $1-2 each; Blue Lagoon 3 is the least crowded)