It depends how long you gonna stay in Malaysia. If you are in Kuala Lumpur, be sure to visit the Twin Towers and also KL Tower. :) If time persist, you may go Cameron Highlands.
You will enjoy stunning views of tea plantations & vegetable farms in Cameron Highlands. :) about 6 years ago
In KL the Petronas Tower are worth a look as well as Chinatown during the day or night. We stayed at the Sol Melia and the area right behind it during the night was pretty lively, I think the street is called Jalan Bukit. Great and cheap massages right on the street.
I going to Borneo... seeing Orangutans (or at least trying to) in Sabah would be my recommendation. about 6 years ago
KL by day:
The Skybridge at the Petronas towers - queue for tickets early in the morning (from about 6 or 7) as they go quickly.
Genting Highlands - casino, resort and cooler climate!
Bukit Tinggi - French themed resort, Malaysia style! About an hour from KL
Shopping, shopping, shopping (The Pavillion, KLCC, Pyramid, The Curve, for electronics Sungei Wang)
KL by night:
Petaling Street in China Town at night and
Changkat - clubs and bars at night
Luna Bar - rooftop pool, bar, club Miami-style
Sipadan Water Village
Borneo - Mount Kinabalu and the orangutans about 6 years ago