Take a hot air balloon ride over the karsts at sunrise

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

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

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

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