The guide will sometimes stop and point out interesting plants but this is an otherwise uninspiring walk through secondary jungle.
Once you get to the caves though things really start to look up. You arrive at the bat observatory (more on which later) and take a small rest before heading up to the caves. Once inside the caves, it is pretty dark so be sure to bring a torch, especially as some of the cave formations are pretty amazing. Highlights of the caves are Adam and Eve's Shower, The Garden of Eden and Abe Lincoln's profile.
Yet more incredible still is the mass exodus of bats from these caves at around 18:00. The clever people at Mulu park have built a bat observatory, which is essentially a semi-circle of seats facing the mouths of the two caves. It is quite possible to sit here until dark to watch the millions of bats streaming out like trails of smoke from the caves. However, if stuff like this bores you or you don't want to walk back to the HQ in the dark, then you can leave before the show is over; just make sure that your guide knows not to have to wait for you.
Walks leave at 14:00 and 14:30 and take 4-5 hours.