A Hostelling International (HI) card is of limited use in Malaysia, as only a handful of places accept it (see www.hihostels.com/destinations/my/hostels for the list). The card can also be used to waive the small initial membership fee at some YMCAs and YWCAs. Bring it if you have one.
An international student identity card (ISIC) is worth bringing. Many student discounts, such as for train travel, are available only for Malaysian students, but some places do offer discounts for international students.