River tubing on the Bohorok is the other classic Bukit Lawang activity. You sit inside an inflated truck tyre and float downstream for two to three hours, through calm stretches of clear water, past jungle-covered banks, and through a few lively sections where the current picks up and requires some steering with your hands.
Tubes and guides are rented through guesthouses for a few dollars. The ride ends a few kilometres downstream, and a waiting vehicle or motorcycle brings you back. It's best to go in the dry season when the river is lower and the rapids are manageable; after heavy rain the river can become too powerful for tubing.