Zipline at the Vang Vieng Challenge or similar adventure parks

Go river tubing down the Nam Song ($10 tube rental + $5-10 at riverside bars)

Kayak the Nam Song River through karst scenery ($10-15 half day)

Explore Tham Chang Cave (entry ~$2) and Tham Phu Kham Cave (with Blue Lagoon 1)

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