The trail from Bario to Pa' Lungan is walkable without a guide. About halfway along you'll see Batu Arit, a large stone featuring bird carvings and humanoid figures with heart-shaped faces.
At Pa’ Lungan is Batu Ritung, a 2m stone table (probably a burial site, although no one is sure). Also near Pa’ Lungan is Perupun, a huge pile of stones of a type assembled to bury the valuables of the dead who had no descendants to receive their belongings.
If you’ve got a bit more time, you could consider basing yourself for a day or two in Pa’ Lungan, believed by many to produce the very best Bario rice.