Labi is a small Iban settlement about 40km south of the coastal road with four longhouses: Rampayoh, Mendaram Besar, Mendaram Kecil and, at the end of the track, Teraja. How much of a longhouse you are able to see if you go without a guide will depend on whether there's an English speaker there to show you around. The cost of such a tour might be B$3 per person. To visit Teraja longhouse, register at the police station nearby.
These longhouses are a mix of the modern and the traditional: you will see women weaving baskets, though nowadays they may be plastic rather than rattan, and the longhouses have 24-hour electricity. Outside, among the fruit trees and clucking chickens, there is a rustic shelter for a row of gleaming cars.
Borneo Guide runs day trips from BSB, with a local lunch arranged. Contact it, or talk to longhouse inhabitants directly, about overnight stays and jungle treks.