After visiting Sadao Customs House, make a plan to see the other sights and activities in Sadao. Wander the forest and lakeside while you're in the area.
Though Sadao Hospital might be what brings you to town, perhaps you'll have a chance to explore more of Sadao while you are here.
Explore the great outdoors at Muang Sadao Municipal Public Park, a lovely green space in Sadao. Make time to visit the temples and shops while you're in the area.