The Genting Highlands, around three quarters of an hour from the city on the East Coast Highway is a purpose built entertainment city a la Las Vegas. With five star hotels, five star attractions and great food and entertainment you could easily spend your whole stay there. Far better though simply to experience it then get back to the real world.
Kuala Selangor, a town north of KL is renowned for the thousands of fire flies which illuminate the nights there. Sit out at one of the many fish restaurants that line the shore there and watch them dance in the sky. Crab Island found in the mouth of the River Klang is a nice place to spend a day, the beaches are pretty but the sea is murky with the sediments from the river.
Finally, the Cameron Highlands are a complete contrast. Around three hour’s drive from KL, they offer beautiful mountainous scenery and cooler, less humid temperatures.