The boat trip along the Sangu River, upstream from Bandarban, is about as exquisitely picturesque and fabulously peaceful as you can possibly get in Bangladesh. Rustic river boats drift down the river to Bandarban from Ruma Bazar (Tk 100, three hours, once every two hours from 8am), accessible from Bandarban's Ruma bus stand. The trip is easier with a guide, but possible (if difficult) independently.
Depending on the condition of the road, buses to Ruma Bazar may terminate at Khokhongjiri (two hours from Bandarban). If this happens, just clamber down to the river past a cluster of tea stalls and small restaurants. At the riverbank, you can either catch a wooden ferry (known as a ‘service boat’) upstream to Ruma Bazar (per person Tk 30, two hours), or downstream back to Bandarban (per person Tk 120, 3½ hours).
From either Ruma Bazar or Khokhongjiri, the lazy ride back to Bandarban is magical, as you pass villages only accessible by boat and enjoy some mesmerising scenery; sometimes rugged, sometimes rural, but always stunning. There may be no service boats, in which case you’ll have to try to pay for your own ‘reserved boat’. Expect to pay at least Tk 3000 from Khokhongjiri, and more from Ruma Bazar. Either way, it’s worth every penny. Note that boats all the way from Ruma Bazar may only operate when the water is at its highest levels, between May and November.