KL and the southern part of the peninsula can be affected by mid summer smog coming up from the Indonesian islands further south.
The island of Langkawi is more rural and considerably quieter, much less commercial, with super beaches at their most popular in our winter and early spring months. Just a handful of hotels and limited restaurant choice / nightlife outside of the resorts.
The peninsula east coast has an even more traditional Malay style, reserved with the main attraction being in the glorious beaches and the totally authentic small villages. This is also the location of Tioman, positioned just off the south east coast, a small island that has a real escapist appeal. This side of the peninsula is at its best April though to October, November to March can see heavy Monsoon rain and high sea swells.
Note - In common with most tropical areas, the beaches in Malaysia are used by tourist and local alike, private beaches are not permitted.
For those in search of rolling hills and planter colonial influenced hotels, try the Cameron Highlands , located roughly half way between KL and Penang and easy to incorporate into a touring itinerary. Another option would be Eco extreme tropical jungle experience of Taman Negara, located just a couple of hours drive north of KL.