Trekking into the national park is the reason most people come to Bukit Lawang. Guides are mandatory inside the park, and the quality of your guide largely determines the quality of your trek — a good one will read the forest, find wildlife that isn't visible to an untrained eye, and get you out safely if the weather turns.
Half-day treks cover the edges of the park and almost always turn up orangutans and Thomas leaf monkeys. Full-day treks go deeper and typically include a river crossing or two. Multi-day treks (usually two or three nights, camping in the forest) are a different experience altogether: you're genuinely in the jungle, far from the village, and the chances of encountering a wider range of animals — gibbons, hornbills, sun bears if you're lucky — rise significantly.
Book through your guesthouse or directly with a licensed guide; the going rate is clearly posted at the park entrance. Bring proper footwear (leeches are common after rain), long sleeves, and more water than you think you need.