The best things to do in Malaysia is none other than fill your stomach with the best food in the region. Since Malaysia is a multi culture and racial country, there are varieties food for you to try. Like the famous nasi lemak, satay, etc from the Malay, Chinese food we have Bak Kut Teh, fried kuay tew etc and Indian, roti canai, capati, etc , the Mamak food is famous with its Nasi Kandar (famous particular in Penang) and lots lots more.
Travelling to another country for a visit, I believe, the main to do is none other than visiting interesting places. If you are heading to the center of the country, Kuala Lumpur, you should be visiting the Twin Towers, which used to be the tallest building in the world, have a walk in the China Town, the Independent Square, The Old Palace or even to the National Musuem if you are intereted in historical and cultural stuffs. Anyway, these places are not far away from each other.
These are just the general stuffs I write about. There are travel blogs that you can read more. Hope that helps.