We took the rainforest canopy walk, which was quite interesting and a very unique experience, but definitely not for those who have a fear of heights!! The walk only lasted about 15 minutes (depending on how slowly you walked), but that was long enough for us, as we walked across the swinging rope bridge between trees!! It was a great experience that helped us learn more about the ecosystem of the rainforest.
I believe you are supposed to call ahead to book the canopy walk, but we were able to show up that day and book it then. There is a fee for the canopy walk (RM5, I think), although entrance to the park is free.
In order to get to FRIM, many people take taxis from Kuala Lumpur, but that is an expensive option, as the taxi drivers will quote you outrageous fares to get there. A better option is to take the train to a nearby station and then take a taxi from there-it's much more reasonable. Also, a visit to FRIM can be combined with a trip to the Batu Caves- they are both in the same area north of Kuala Lumpur. Just take a taxi from FRIM to the Batu Caves (or vice versa). We also discovered (by asking the gate guard at FRIM) that there is a local mall where we could get lunch. Otherwise, there wasn't much available nearby (and the cafeteria at FRIM was closed).