Lawachara National Park covers a small but important patch of semi-evergreen forest between Sylhet and Srimangal. The hoolock gibbons are the headline attraction — early-morning treks (5am start) into the forest with a park guide are the best chance of hearing their loud whooping calls and, if you're lucky, seeing a small family group in the canopy.
Beyond the gibbons, the park also has capped langurs, giant squirrels, and a long list of birds. Three short trails leave from the entrance gate; the longest takes about three hours. Local guides at the gate are essential — both for finding wildlife and for staying on the right trails — and very inexpensive.