I'm not sure I'd recommend tropical rainforest for those looking for a traditional walking experience; there are no views, the terrain is horribly difficult to walk through, there are hordes of nasty beasts waiting to attack you, it rains constantly and the humidity makes any exercise challenging, but one thing's for sure: you'll come out the other end with some great tales and a serious sense of achievement, and that's why I reckon that rainforest walking is something every serious walker should try at least once.
Taman Negara in central Malaysia is the oldest tropical rainforest in the world, weighing in at an astonishing age of 130 million years, and the park has some excellent walking opportunities. There are longer walks into the forest, but these require guides as it's so easy to get lost; you can, however, head off to the fishing lodge of Kuala Perkai by simply buying the right permit, and for most of us it's easily remote enough.
Rainforests are not for the squeamish, though. You will come across countless insects, leeches galore, meat-eating ants, mosquitoes, spiders and plenty of other nasty specimens. The humidity is one of the most uncomfortable feelings in the world; nothing you own will be dry for the entire walk, and you may well get sore patches from walking in these conditions. Add in swollen rivers, fallen trees, huge amounts of rain and kilometre markers that are economical with the truth, and you have a frustrating experience that's surprisingly rewarding. This might be a relatively short walk, but it's quite an achievement; if you've conquered mountains, valleys and mountains, I highly recommend a stab at the rainforest. Just don't say I didn't warn you!