There are various beautiful national parks in Malaysia. There are many different types of expeditions available, ranging from those where you hardly lose sight of the hotel to those were you are fully immersed in the jungle with only the guide and yourself if you are willing to pay the money!
Tours vary from about 4 days to 2 weeks or more. It is very unlikely in most of the national parks for you to see a tiger or an elephant as it is really likely only if you are going to be staying for longer than a few days, i.e., for a couple of weeks at least. One of the most common forms of wildlife that you will encounter in the jungle however are definitely leeches! In the rainforest it is very very humid but actually it is not incredibly hot. This is because of the large amount of shade afforded by the canopy created by the interlocking trees.
Shop around for deals of getting into the jungle and make your decision based on what type of person you are. If you are going to enjoy a lot of hiking without seeing any other people for days or even weeks then you can have that choice, alternatively you can have a much more 'packaged' tour in which you will probably stay in a very built up tourist town which has probably just grown out of the demand for people wanting to stay in the jungle.
To escape from the muggy tropics, do as the English did and head up to the cooler highlands of West Malaysia.
There is a zoo at Taiping.